The Prohbitin homology domain (PHB d) is found in a large number of membrane-embedded proteins (currently 1780 in the SMART database) in organisms ranging from bacteria to mammals. PHB-d proteins have been found in association with ER, mitochondria and cell membranes; they have been implicated in a wide variety of processes including control of cell proliferation, melanogenesis, osmotic homeostasis, and mechanosensation. Previous work (Huber et al., PNAS 103:7019-17086, 2006) has shown that two non-homologous PHB-d proteins, mammalian Podocin and worm MEC-2, function similarly in different systems. Both bind and regulate membrane proteins (TRPC and DEG/ENaC ion channels respectively), homo-oligomerize, and bind cholesterol. These properties led to the hypothesis that PHB-d proteins regulate membrane bound proteins by controlling their lipid environment. To test whether other PHB-d proteins share these characteristics, we have begun to characterize the complete family of C. elegans PHB-d proteins. Thirteen worm proteins contain PHB domains: MEC-2, STO-1-6, STL-1, UNC-1, UNC-24, PHB-1, PHB-2 and C42C1.15. Eight of the worm proteins (MEC-2, STO-1-6 and UNC-1) are closely related to mammalian Stomatin, a PHB-d protein found in many cell types including sensory neurons. Three (UNC-1, UNC-24 and MEC-2) have been previously shown to be expressed in neurons and to regulate ion channels. We generated GFP promoter fusions to characterize the expression of the remaining genes. The sto genes are expressed almost exclusively in neurons, suggesting that they could regulate ion channels or other membrane proteins. Most sto genes (
sto-1,
sto-2,
sto-3 and
sto-5) are expressed in only a few cells;
sto-4 is expressed in many neurons and
sto-6 may be pan-neuronal. Single knockout strains of the 6 sto genes display no apparent phenotype; they may be redundant or exert subtle effects. The PHB-d genes known to function in mitochondria (
phb-1,
phb-2 and
stl-1) are expressed widely, including in pharynx and body wall muscle where
phb-1 and
phb-2 have been shown to be important for mitochondrial morphology. We are currently testing oligomerization and cholesterol binding of these proteins.