Together with sorf-2 negatively regulates the levels of phosphatidylinositol 3-phosphate (PtdIns3P) to enable the conversion of early endosomes to late endosomes. Binds to sorf-2 and the sorf-1-sorf-2 complex likely acts through bec-1, a non-catalytic subunit of phosphatidylinositol 3-kinase (PI3K), to suppress PI3K activity, thereby negatively regulating endosomal PtdIns3P levels. Interacts with sorf-2; the interaction is direct. Interacts with bec-1. Coelomocytes contain larger early and late endosomes enriched with PtdIns3P. Delayed conversion of early endosomes to late endosomes with early endosomes retaining PtdInsP3 for a longer duration of time which may possibly be due to a delay in either the turnover or transport of PtdIns3P out of the endosome. This leads to continuous fusion of early endosomes which continues until rab-5 is displaced and rab-7 is recruited. Double knockout with sorf-2 results in a similar phenotype as the individual single sorf-1 knockout. Double knockout with bec-1 results in smaller endosomes and an irregular distribution pattern of PtdIns3P in the cytoplasm. Double knockout with vps-11, vps-18 or vps-39, subunits of the CORVET/HOPS complex, results in larger endosomes and larger lysosomes and thus suppresses the endosome/lysosome fusion defects in coelomocytes of the individual vps-11, vps-18 and vps-39 single mutants. Likewise, RNAi-mediated knockdown in a vps-41 mutant background (a subunit of the CORVET/HOPS complex) also suppresses the endosome/lysosome fusion defects in the vps-41 single mutant. However, double knockout with the CORVET/HOPS complex subunits vps-16 or vps-33.1, does not suppress the endosome/lysosome fusion defects in coelomocytes of the individual vps-16 and vps-33.1 single mutants. Double knockout with proteins involved in rab-5 to rab-7 switching in early to late endosome conversion such as rab-7, sand-1 and tbc-2 results in enlarged vacuoles, delayed endosomal cargo transport and persistent PtdIns3P in early endosomes in coelomocytes. Belongs to the WD repeat WDR91 family.