Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids
ReviewPAT proteins, an ancient family of lipid droplet proteins that regulate cellular lipid stores
Introduction
Lipid droplets are intracellular storehouses of lipid esters. In organisms as diverse as fungi, plants, insects, and mammals, lipid droplets sequester lipids inside cells for later use as metabolic fuel, membrane components, post-translational protein modifications, and signaling molecules [1]. Investigations over the past decade have revealed lipid droplets as regulated organelles of surprising complexity. Lipid droplets contain a core of neutral lipid surrounded by a phospholipid monolayer and coated by specific proteins, including proteins of the PAT family that are the subject of this review. Proteomic studies have identified lipid droplet-associated proteins that are involved in lipid metabolism and transport, intracellular trafficking, signaling, chaperone function, RNA metabolism, and cytoskeletal organization [2], [3], [4], [5], [6], [7]. The protein coat of lipid droplets can vary between droplets within a cell, between metabolic conditions, and between cell types. This heterogeneity of the protein coat is consistent with the dynamic changes in morphology and intracellular location that lipid droplets undergo according to the metabolic state or developmental stage of the cell or organism. Lipid droplets and lipid droplet proteins have been the subject of multiple recent reviews [8], [9], [10], [11], [12], [13], [14], including many articles within this issue.
The spark that ignited interest in mammalian lipid droplets was the discovery by the laboratory of Constantine Londos of perilipin as a lipid droplet protein phosphorylated in response to signals that stimulated breakdown of triacylglycerol (TAG) stored in fat cells (adipocytes) [15], [16], [17]. This discovery pointed to the existence of regulatory mechanisms to control fat storage. Subsequent work established perilipin as the gatekeeper of the adipocyte lipid storehouse. Studies in cells and in mice have shown that perilipin can either restrict lipid droplet access of enzymes that hydrolyze lipid esters (lipases) or facilitate their enzymatic activity under appropriate metabolic conditions.
Perilipin is the founding member of a family of proteins that share sequence similarity and the ability to bind lipid droplets (Fig. 1). When first grouped as a family [18], these proteins were termed the “PAT family” after the first three members identified: perilipin, adipocyte differentiation-related protein (ADRP), and tail-interacting protein of 47 kDa (TIP47). Two other proteins, S3–12 [19], [20] and OXPAT [21], [22], [23], round out the family in mice and humans. The members of the family differ from one another in size, tissue expression, affinity for lipid droplets, stability when not bound to lipid droplets, and transcriptional regulation. These differences imply that each PAT protein has distinct cellular functions, but all PAT proteins likely regulate the interface between lipid droplets and their cellular environment. The PAT proteins divide conceptually into those that are expressed in a tissue-restricted manner (perilipin, S3–12, and OXPAT) versus ubiquitous manner (ADRP and TIP47) and into those that are constitutively bound to lipid droplets (perilipin and ADRP, the CPATs) versus those that demonstrate exchangeable lipid droplet binding (TIP47, S3–12, and OXPAT, the EPATs) [24]. The next four sections of this review examine the mammalian PAT proteins in detail, with emphasis on the best-characterized member of the family, perilipin. Other reviews specifically focused on the PAT family have appeared over the past few years [24], [25], [26]. For this review our goal is to incorporate important new published findings into a comprehensive overview of this increasingly studied family of proteins.
The PAT family proteins have been independently identified, cloned, and characterized by many different labs world-wide, each lab working in different model systems. Accordingly, each PAT protein goes by several different names in the literature, which are summarized in Table 1. It is likely that in the future a consensus nomenclature for the PAT family will be agreed upon by the lipid droplet community. For the purpose of clarity in this review, we have chosen to use the names perilipin, ADRP, TIP47, S3–12, and OXPAT to refer to the mammalian proteins.
The PAT family is evolutionarily ancient; family members are present in many animal species (like frogs and flies) and even in fungi and slime molds [18], [27], [28], [29]. This conservation of the family points to the importance of its function in regulating intracellular lipid stores. Indeed, significant conservation between mammals and insects with respect to the molecular mechanisms of lipid droplet metabolism extends to other proteins, including receptors, enzymes, and co-factors, at the level of sequence and/or function (Fig. 2). This conservation highlights the tremendous potential of using genetically tractable model organisms to discover novel mechanisms for the control of intracellular lipid storage and utilization. Accordingly, this review includes an account of recent discoveries made in non-mammalian systems.
Aberrant cellular lipid metabolism has been associated with common human conditions that cause significant morbidity and mortality, including atherosclerosis, cardiomyopathy, obesity, type 2 diabetes mellitus, and non-alcoholic fatty liver disease. We anticipate that basic discoveries about how PAT proteins, mammalian and non-mammalian, contribute to the regulation of cellular lipid stores will lead to novel approaches to these pressing issues in human health.
Section snippets
Perilipin
In mice and humans, a single perilipin gene gives rise to at least three protein isoforms (perilipin A, B and C) that share a common N-terminal region and differ in their C-terminal tails [30] (Fig. 1). The shared region encompasses the PAT domain, three of the 6 recognized protein kinase A (PKA) sites, a stretch of 16 aspartate and glutamate residues (the acidic loop region), and two of three hydrophobic regions that target the protein to lipid droplets [31]. These regions have been
ADRP/adipophilin/ADFP/ADPH
ADRP is a 50 kDa protein that was first identified as an RNA transcript significantly induced during differentiation of cultured adipocytes [112], [113]. Sequence similarity of ADRP to the perilipins led to the discovery that ADRP coats small lipid storage droplets in a variety of cell lines, including early differentiating 3T3-L1 adipocytes [114].
TIP47, the first reported exchangeable lipid droplet protein
Tail-interacting protein of 47 kDa (TIP47) was identified as a relative of perilipin and ADRP by sequence similarity of their amino-termini [116], [147] (Fig. 1). Whereas perilipin and ADRP are both regulated by PPARs as discussed above, TIP47 is not PPAR-regulated [36]. In contrast to perilipin and similar to ADRP, TIP47 is expressed in almost if not all tissues. The sequence homology between ADRP and TIP47 extends throughout the entire lengths of the proteins, over which they are 43%
S3–12 and OXPAT, tandem genes with reciprocal expression
Like TIP47, the most recent additions to the PAT proteins, S3–12 and OXPAT, are EPATs (Fig. 1). S3–12 and OXPAT are encoded by tandem genes on human chromosome 19 and mouse chromosome 17 [22], [23]; the gene for TIP47, the only other mammalian EPAT, is located less than 200,000 base pairs upstream. Despite their proximity in the genome, the genes for S3–12 and OXPAT appear to have evolved for specialized functions based on their distribution in mouse tissues. S3–12 is expressed primarily in the
Non-mammalian PAT proteins
PAT proteins are not restricted to mammals. Proteins with significant sequence similarity have been found in many other animal species (ranging from sea anemones and insects to snails, frogs and sea urchins) and even in unicellular eukaryotes (such as choanoflagellates, fungi, and slime molds) ([18], [27], [28], [29] and MAW, unpublished observations). Functional data have been reported only from a few species, but what is available suggests that across this broad phylogenetic spectrum PAT
Mammalian and non-mammalian PAT proteins, an ancient family for a fundamental need
The discovery of perilipin opened a window into the control of cellular lipid metabolism that many investigators in many fields have now climbed through. These studies have established the PAT family as coat proteins for lipid droplets that regulate access to the lipid ester core. A theme that runs throughout studies of the PATs is that they protect the stored lipid esters from uncontrolled hydrolysis by cytoplasmic lipases. Each of the 5 mammalian PAT proteins likely is specialized for
Acknowledgements
Our studies of PAT proteins and lipid droplets are supported by NIH grants DK068046 to PEB and GM64687 and AG031531 to MAW.
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