The amino acid sequences of 40 secreted phospholipase A2's (PLA2) were aligned and a phylogenetic tree derived that has three main branches corresponding to elapid (group I), viperid (group II), and insect venom types of PLA2. The human pancreatic and recently determined nonpancreatic sequences in the comparison align with the elapid and viperid categories, respectively, indicating that at least two PLA2 genes existed in the vertebrate line before the divergence of reptiles and mammals about 200-300 million years ago. This allows resolution for the first time of major genetic events in the evolution of current PLA2's and the relationship of human PLA2's to those of snake venom, many of which are potent toxins. Implications for possible mechanisms of regulation of mammalian intra- and extracellular PLA2's are discussed, as well as issues relating to the search for the controlling enzymes in arachidonic acid release, prostaglandin generation, and signal transduction.

Apoptosis is a gene-directed form of cell death that is essential for normal development and health. Yet abnormally high levels of apoptosis are linked to many degenerative diseases. Some important biochemical events in apoptosis have been identified, but the therapeutic utility of blocking cell death remains unclear. An important question in this regard is whether cells rescued from apoptosis can function. We have investigated the mechanism of cell death in two Drosophila mutant strains that exhibit age-related retinal degeneration. One of these mutations also occurs in humans, where it causes retinitis pigmentosa. We found that retinal cell death in rdgC and ninaE(RH27)/+ flies occurred by apoptosis and was blocked by eye-specific expression of the baculoviral cell survival protein p35. Most importantly, the mutant flies expressing p35 showed significant retention of visual function. The results demonstrate a therapeutic benefit of late-stage inhibition of apoptosis to flies, and suggest that similar results may be obtained in higher organisms.

The disulfide bond between Cys-110 and Cys-187 in the intradiscal domain is required for correct folding in vivo and function of mammalian rhodopsin. Misfolding in rhodopsin, characterized by the loss of ability to bind 11-cis-retinal, has been shown to be caused by an intradiscal disulfide bond different from the above native disulfide bond. Further, naturally occurring single mutations of the intradiscal cysteines (C110F, C110Y, and C187Y) are associated with retinitis pigmentosa (RP). To elucidate further the role of every one of the three intradiscal cysteines, mutants containing single-cysteine replacements by alanine residues and the above three RP mutants have been studied. We find that C110A, C110F, and C110Y all form a disulfide bond between C185 and C187 and cause loss of retinal binding. C185A allows the formation of a C110-C187 disulfide bond, with wild-type-like rhodopsin phenotype. C187A forms a disulfide bond between C110 and C185 and binds retinal, and the pigment formed has markedly altered bleaching behavior. However, the opsin from the RP mutant C187Y forms no rhodopsin chromophore.Early work indicated the important role of the intradiscal domain in folding in vivo and function of rhodopsin (1). The presence of a disulfide bond between Cys-110 and Cys-187 was established (2) and it was shown later to be required for optimal signal transduction (3). A disulfide bond between two cysteines in the extracellular domain at positions equivalent to the above two cysteines in rhodopsin is known now to be conserved in most of the known G-protein-coupled receptors (4, 5). The presence of a disulfide bond indicated a tertiary structure in the intradiscal domain of rhodopsin. Strong support for this conclusion was forthcoming from extensive designed mutagenic studies as well as from studies of a large number of naturally occurring point mutations in rhodopsin that are associated with retinitis pigmentosa (RP) (6-11). Mutations in the intradiscal domain were shown to cause total or partial misfolding in vivo in rhodopsin, misfolding being operationally defined as the loss of ability to bind 11-cis retinal (6). Further, evidence was presented that misfolding results from the formation of a disulfide bond in the intradiscal domain different from the normal Cys-110-Cys-187 disulfide bond (12). Recently, Oprian and colleagues demonstrated the formation of a disulfide bond under certain conditions between Cys-185 and Cys-187 in a rhodopsin mutant reconstituted from two appropriate rhodopsin fragments (13).Further studies showed that although replacements of Cys-110 and Cys-187 by serine residues prevented folding of rhodopsin to a functional structure, corresponding replacements by alanine residues allowed the formation of the correctly folded dark-state structure. This result paralleled the earlier findings on the small-protein bovine pancreatic trypsin inhibitor, and, by analogy, suggested a globular structure for the intradiscal domain in rhodopsin (3). Recently, point mutations at the intradiscal cys...

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