Originally identified as neuronal protein that specifically associates with HTT/huntingtin and the binding is enhanced by an expanded polyglutamine repeat within HTT possibly affecting HAP1 interaction properties. Both HTT and HAP1 are involved in intracellular trafficking and HAP1 is proposed to link HTT to motor proteins and/or transport cargos. Seems to play a role in vesicular transport within neurons and axons such as from early endosomes to late endocytic compartments and to promote neurite outgrowth. The vesicular transport function via association with microtubule-dependent transporters can be attenuated by association with mutant HTT. Involved in the axonal transport of BDNF and its activity-dependent secretion; the function seems to involve HTT, DCTN1 and a complex with SORT1. Involved in APP trafficking and seems to facilitate APP anterograde transport and membrane insertion thereby possibly reducing processing into amyloid beta. Involved in delivery of gamma-aminobutyric acid (GABA(A)) receptors to synapses; the function is dependent on kinesin motor protein KIF5 and is disrupted by HTT with expanded polyglutamine repeat. Involved in regulation of autophagosome motility by promoting efficient retrograde axonal transport. Seems to be involved in regulation of membrane receptor recycling and degradation, and respective signal transduction, including GABA(A) receptors, tyrosine kinase receptors, EGFR, IP3 receptor and androgen receptor. Among others suggested to be involved in control of feeding behavior (involving hypothalamic GABA(A) receptors), cerebellar and brainstem development (involving AHI1 and NTRK1/TrkA), postnatal neurogenesis (involving hypothalamic NTRK2/TrkB), and ITPR1/InsP3R1-mediated Ca(2+) release (involving HTT and possibly the effect of mutant HTT). Via association with DCTN1/dynactin p150-glued and HTT/huntingtin involved in cytoplasmic retention of REST in neurons. May be involved in ciliogenesis. Involved in regulation of exocytosis. Isoform A but not isoform B seems to be involved in formation of cytoplasmic inclusion bodies (STBs). In case of anomalous expression of TBP, can sequester a subset of TBP into STBs; sequestration is enhanced by an expanded polyglutamine repeat within TBP. Self-associates. Interacts with HTT/huntingtin; enhanced by an expanded polyglutamine repeat within HTT. Isoform A interacts with DCTN1; decreased in presence of HTT with expanded polyglutamine repeat; decreased by phosphorylation of Hap1 isoform A at Thr-598. Isoform A interacts with KLC2; decreased by phosphorylation of Hap1 isoform A at Thr-598. Isoform A interacts with ITPR1 and APP. Isoform A interacts with AR; decreased by an expanded polyglutamine repeat within AR. Isoform A interacts with YWHAZ; enhanced by phosphorylation of Hap1 isoform A at Thr-598. Isoform A interacts with BDNF and SORT1; probably forming a complex involved in proBDNF trafficking, degradation and processing. Interacts with TBP, AHI1, HGS and KALRN. Interacts with KIF5A, KIF5B, KIF5C and GABRB3; indicative for an HAP1:KIF5 complex transporting a GABA(A) receptor as cargo. Interacts with ATXN3; in STBs. Interacts with NTRK2; HAP1 stabilizes association of NTRK2 with SORT1 preventing NTRK2 degradation. Interacts with CCDC113. Localizes to large nonmembrane-bound cytoplasmic bodies found in various types of neurons, called stigmoid bodies (STBs); STB formation is regulated by the ratio of isoform A to isoform B (PubMed:9798945). In the nucleus localizes to nuclear rods. In NGF-stimulated PC2 cells isoform A can move anterogradely fom neurite cell body to neurite terminal and is localized to growth cone tips whereas isoform B stays in the cell body. In NGF-stimulated PC2 cells isoform A can move anterogradely fom neurite cell body to neurite terminal and is localized to growth cone tips whereas isoform B stays in the cell body (PubMed:10924259). Localization to neuronal processes and neurite tips is decreased by YWHAZ (PubMed:17166838). In the brain, especially in the olfactory bulb and in the brain stem. No detectable expression in peripheral tissues such as lung, testis, spleen, and small intestine. Isoform A is phosphorylated on Thr-598.