Dictvostelium development is regulated by response to secreted cAMP. We have isolated genes for 4 cAMP receptor subtypes and determined their role during the Dictvostelium developmental cycle. Each has a distinct affinity for cAMP which correlates well with the relative concentrations of extracellular cAMP present at the time of their expression. Dictvostelium specifically disrupted for each gene exhibit developmental abnormalities consistent with their cAMP affinities and temporal and spatial patterns of expression. Thus, the multiple responses of cells to cAMP can be attributed to distinct receptor subtypes encoded by unique genes. Nonetheless, current data suggest that cAMP receptors, other than CARs 1-4, play a role during Dictvostelium development. Mutated Dictvostelium are still able to regulate gene expression by cAMP-dependent event. We have now isolated several genes which share weak sequence homology with the known cAMP receptor genes across their entire coding regions. These genes potentially encode a new set of cAMP receptors which may mediate cAMP regulated gene expression. Alternatively, they may interact with a completely different ligand; several other G-protein-linked responses have been described in Dictvostelium. The promoters which regulate the expression of the cAMP receptor genes have been identified. Deletion studies indicate that they may be under both positive and negative control and in some cases we have localized specific DNA elements that are absolutely required for developmentally regulated expression. Interestingly, the cAMP receptors themselves are regulated by response to extracellular cAMP. In particular, subtype 1 uses a dual promoter system. Each is active at a distinct developmental stage and responsive to a different signalling mechanism. The promoters have been fused with reporter genes that permit the detection of cell-type specific expression patterns. Consistent with our previous studies of mRNA localization, the promoters exhibit specific patterns of localized expression. The mammalian adipocyte represents another excellent system for the study of differentiation. We have primarily focussed on the structure and function of perilipin, an adipocyte-specific, phosphoprotein that is localized at the periphery of lipid droplets. We have isolated cDNA and genomic sequences for perilipin from rat and mouse. Molecular analyses predicted the existence of two perilipin protein forms that result from alternative RNA splicing. These protein forms have been confirmed. Expression in sense and antisense orientations is being used to examine the function of perilipin in lipid metabolism.
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