Component of the FTS/Hook/FHIP complex (FHF complex) (PubMed:18799622, PubMed:32073997). The FHF complex may function to promote vesicle trafficking and/or fusion via the homotypic vesicular protein sorting complex (the HOPS complex) (PubMed:18799622). FHF complex promotes the distribution of AP-4 complex to the perinuclear area of the cell (PubMed:32073997). Required for spermatid differentiation. Probably involved in the positioning of the microtubules of the manchette and the flagellum in relation to the membrane skeleton (By similarity). Self-associates (PubMed:18799622, PubMed:32073997). Component of the FTS/Hook/FHIP complex (FHF complex), composed of AKTIP/FTS, FHIP1B, and one or more members of the Hook family of proteins HOOK1, HOOK2, and HOOK3 (PubMed:18799622, PubMed:32073997). Interacts directly with AKTIP/FTS, HOOK2 and HOOK3 (PubMed:18799622, PubMed:32073997). Associates with several subunits of the homotypic vesicular sorting complex (the HOPS complex) including VPS16, VPS18, VPS39 and VPS41; these interactions may be indirect (PubMed:18799622). Interacts with CCDC181 (By similarity). Interacts (via coiled-coil region) with RIMBP3 (via C-terminus) (By similarity). Interacts with LRGUK (via guanylate kinase-like domain) (By similarity). Interacts with microtubules (PubMed:11238449). May interact with CLN3 (PubMed:15471887). Interacts with AP4M1; the interaction is direct, mediates the interaction between FTS-Hook-FHIP (FHF) complex and AP-4 and the perinuclear distribution of AP-4 (PubMed:32073997). Localizes to punctate cytoplasmic foci which do not appear to overlap with early or late endosomes, the endoplasmic reticulum, multivesicular bodies (MVBs), lysosomes, or mitochondria (By similarity). Often found in close association with microtubules (By similarity). Does not associate with the Golgi complex. During spermiogenesis, it localizes to the manchette in spermatids from steps 8-10. It is also present between the microtubule manchette and the nucleus. During manchette elongation, it is preferentially localized to the nuclear ring of the manchette, whereas the strong localization to the manchette decreases. In more mature spermatids, while the manchette migrates posteriorly, it localizes to punctuates spots. At later stages of spermatid differentiation, the punctuate expression pattern is found at both the attachment site and the proximal end of the elongated manchette. In contrast, it is not present in mature spermatozoa (By similarity). Belongs to the hook family.