Huntingtin-associated protein 1 (HAP1) is a binding partner of the
huntingtin protein responsible for Huntington’s disease, with postulated roles
in vesicle trafficking and microtubule transport. The current research provides
the first physiological evidence supporting a role for HAP1 in regulation of
both exocytosis and endocytosis. Amperometry, utilising chromaffin cells from
both WT and HAP1 KO mice, was used to investigate the role of HAP1 in
exocytosis. There was a significant reduction in the number of exocytosis events
seen in cells from HAP1 KO animals, compared with WT cells. Whilst
there were no significant changes in spike parameters the pre-spike foot signal
was prolonged in HAP1 KO cells, suggesting that HAP1 may play a role in fusion
pore stabilisation. A reduction in the size of the readily releasable pool of
vesicles was also observed in HAP1 KO cells, consistent with
a reduction in the number of morphologically docked vesicles seen in these
cells using electron microscopy. The role of HAP1 in endocytosis was also
investigated using patch clamp capacitance measurement, with this data showing a
reduced rate of membrane retrieval in the HAP1 KO cells compared to WT. Using a
proteomics approach we identified several novel HAP1 interactions with proteins
involved in exocytosis, endocytosis and protein trafficking and the mislocalisation
of one of these proteins in HAP1 KO neurons. These results therefore suggest,
for the first time, the involvement of HAP1 in regulation of both exocytosis
and endocytosis, as well as providing mechanisms in which this regulation might
be accomplished based on newly identified protein interactions.