Previous studies have shown that UNC-53 is required for the migration of cells and the outgrowth of several neuronal processes. UNC-53 binds the adapter protein SEM-5 (Stringham et al., 2002) as well as ABI-1, a regulator of Arp 2/3 mediated actin filament assembly (Schmidt et al., 2009). A two hybrid screen identified the T12G3.1 gene product as a strong interactor of UNC-53. RNAi and mutant analysis of T12G3.1 revealed a partial overlap in cell migration phenotypes with
unc-53. While the outgrowth of the posterior excretory canals is truncated in both T12G3.1 and
unc-53 mutants, the latter has a guidance phenotype in mechanosensory neurons that does not appear to be shared with T12G3.1. T12G3.1 encodes a protein of 693 amino acids that contains a zinc finger of the ZZ type, a SH3 binding domain, and a UBA (ubiquitin associated) domain. T12G3.1 is 38 % identical to murine
p62/sequestosome-1, especially around the ZZ and UBA domains. In mammals,
p62 has been shown to shuttle polyubiquitinated proteins to the proteasome for degradation and to bind autophagocytic vesicles, suggesting a role in intracellular trafficking. Consistent with this hypothesis, a pT12G3.1::gfp fusion is expressed in cells that have specialized roles in secretion or endocytosis including the excretory gland cell, motorneurons, coelomocytes, intestine, and spermatheca. A translational pT12G3.1::T12G3.1::GFP fusion shows localized expression to cell bodies but is excluded from neuronal processes. We observe persistent expression of UNC-53 in neurons and in coelomocytes, specialised cells that endocytose pseudocoelomic fluid suggesting that UNC-53 may also have a role in vesicle trafficking. Using an assay where GFP produced in body muscle is secreted into the pseudocoelom and then endocytosed into coelomocytes, we have demonstrated a coelomocyte-uptake (cup) defect in worms depleted of T12G3.1,
unc-53, or
arx-2/Arp2 by RNAi and in genetic nulls. Similarly, null alleles of
unc-53 are defective in receptor mediated endocytosis of yolk protein in oocytes.
p62/sequestosome-1 is a physical interactor and regulator of the
pmk-1/p38 MAPK involved in innate immunity to microbial pathogens. We find that, like
pmk-1 mutants, null alleles of
unc-53 and T12G3.1 are sensitive to Pseudomonas aeruginosa and are both expressed in intestine, the primary site of
pmk-1 activity. Thus UNC-53 appears to have a multi functional role in cell migration, protein trafficking, and may be involved in innate immunity. We propose a model where UNC-53 may mediate all of these activities by modulating cytoskeletal dynamics.