The Caenorhabditis elegans unc-32 gene encodes alternative forms of a vacuolar ATPase a subunit.

Eukaryotes possess multiple isoforms of the a subunit of the V(0) complex of vacuolar-type H(+)-ATPases (V-ATPases). Mutations in the V-ATPase a3 isoform have recently been shown to result in osteopetrosis, a fatal disease in humans, but no function has yet been ascribed to other isoforms. In Caenorhabditis elegans, the unc-32 mutant was originally isolated on the basis of its movement defect. We have isolated four new mutant alleles, the strongest of which is embryonic lethal. We show here that unc-32 corresponds to one of the four genes encoding a V-ATPase a subunit in the nematode, and we present their expression patterns and a molecular analysis of the gene family. unc-32 gives rise via alternative splicing to at least six transcripts. In the uncoordinated alleles, the transcript unc-32 B is affected, suggesting that it encodes an isoform that is targeted to synaptic vesicles of cholinergic neurons, where it would control neurotransmitter uptake or release. Other isoforms expressed widely during embryogenesis are mutated in the lethal alleles and would be involved in other acidic organelles. Our results indicate that V-ATPase a subunit genes are highly regulated and have tissue-specific function.

Conformational specificity of the chaperonin GroEL for the compact folding intermediates of alpha-lactalbumin.

The chaperonin GroEL binds unfolded polypeptides, preventing aggregation, and then mediates their folding in an ATP-dependent process. To understand the structural features in non-native polypeptides recognized by GroEL, we have used alpha-lactalbumin (alpha LA) as a model substrate. alpha LA (14.2 kDa) is stabilized by four disulfide bonds and a bound Ca2+ ion, offering the possibility of trapping partially folded disulfide intermediates between the native and the fully unfolded state. The conformers of alpha LA with high affinity for GroEL are compact, containing up to three disulfide bonds, and have significant secondary structure, but lack stable tertiary structure and expose hydrophobic surfaces. Complex formation requires almost the complete alpha LA sequence and is strongly dependent on salts that stabilize hydrophobic interactions. Unfolding of alpha LA to an extended state as well as the burial of hydrophobic surface upon formation of ordered tertiary structure prevent the binding to GroEL. Interestingly, GroEL interacts only with a specific subset of the many partially folded disulfide intermediates of alpha LA and thus may influence in vitro the kinetics of the folding pathways that lead to disulfide bonds with native combinations. We conclude that the chaperonin interacts with the hydrophobic surfaces exposed by proteins in a flexible compact intermediate or molten globule state.

Disulfide-rearranged molten globule state of alpha-lactalbumin.

A three-disulfide form of human alpha-lactalbumin, with free thiols on Cys6 and Cys120, can adopt the molten globule conformation. It then spontaneously rearranges its three disulfide bonds to many isomers that tend to maintain the molten globule conformation. The distribution of free thiol groups within the rearranged species has been determined quantitatively by chemical modification and peptide mapping. The protein's eight cysteine residues were modified with nearly equal frequencies, although there were significant departures from randomness. The results confirm that the molten globule state of alpha-lactalbumin does not maintain the native-like topology of the polypeptide backbone but is more like a collapsed form of an unfolded protein.

Pathway of disulfide-coupled unfolding and refolding of bovine alpha-lactalbumin.

alpha-Lactalbumin's four disulfide bonds have been used to probe the nature of its native, molten globule, and unfolded states. The Ca2+ concentration could be used to vary the stability of the native state because it binds a single calcium ion with high affinity. In the Ca(2+)-bound native state, the Cys6-120 and Cys28-111 disulfides were reduced sequentially, reflecting their relative accessibilities. The native disulfides do not stabilize the native conformation as much as in other proteins, nor was the unfolding process as cooperative. After two disulfides were reduced, only part of the protein molecule appeared to remain folded. In the absence of Ca2+, the native state was much less stable, and a molten globule-type conformation tended to be adopted. Formation of disulfide bonds in the reduced protein was initially noncooperative and nonspecific. The molten globule state increased the rate at which disulfides were formed, by tending to keep close in space cysteine residues that are distant in the sequence. A large number of nonnative disulfide pairings were preferred, and the disulfides were not stabilized to any great extent. The alpha-lactalbumin molten globule seems to be stabilized by nonspecific interactions and without a well-defined topology. The native protein was regenerated only when Ca2+ bound to and stabilized the three-disulfide intermediate lacking the Cys6-120 disulfide, which then rapidly formed the fourth disulfide bond. The rate-limiting intramolecular steps were rearrangement of disulfide bonds to native combinations in species with two and three disulfide bonds.