In the nematode Caenorhabditis elegans, there exist two &mgr;1 medium chains of the AP-1 clathrin-associated protein complex. Mutations of
unc-101, the gene that encodes one of the &mgr;1 chains, cause pleiotropic effects (). In this report, we identified and analyzed the second
mu1 chain gene,
apm-1. Unlike the mammalian homologs, the two medium chains are expressed ubiquitously throughout development. RNA interference (RNAi) experiments with
apm-1 showed that
apm-1 and
unc-101 were redundant in embryogenesis and in vulval development. Consistent with this, a hybrid protein containing APM-1, when overexpressed, rescued the phenotype of an
unc-101 mutant. However, single disruptions of
apm-1 or
unc-101 have distinct phenotypes, indicating that the two medium chains may have distinct functions. RNAi of any one of the small or large chains of AP-1 complex (sigma1, beta1, or gamma) showed a phenotype identical to that caused by the simultaneous disruption of
unc-101 and
apm-1, but not that by single disruption of either gene. This suggests that the two medium chains may share large and small chains in the AP-1 complexes. Thus,
apm-1 and
unc-101 encode two highly related &mgr;1 chains that share redundant and distinct functions within AP-1 clathrin-associated protein complexes of the same tissue.