A Single ß Adaptin Contributes to AP1 and AP2 Complexes and Clathrin Function in Dictyostelium

The assembly of clathrin-coated vesicles is important for numerous cellular processes, including nutrient uptake and membrane organization. Important contributors to clathrin assembly are four tetrameric assembly proteins, also called adaptor proteins (APs), each of which contains a beta subunit. We identified a single beta subunit, named beta 1/2, that contributes to both the AP1 and AP2 complexes of Dictyostelium. Disruption of the gene encoding beta 1/2 resulted in severe defects in growth, cytokinesis and development. Additionally, cells lacking beta 1/2 displayed profound osmoregulatory defects including the absence of contractile vacuoles and mislocalization of contractile vacuole markers. The phenotypes of beta 1/2 null cells were most similar to previously described phenotypes of clathrin and AP1 mutants, supporting a particularly important contribution of AP1 to clathrin pathways in Dictyostelium cells. The absence of beta 1/2 in cells led to significant reductions in the protein amounts of the medium-sized subunits of the AP1 and AP2 complexes, establishing a role for the beta subunit in the stability of the medium subunits. Dictyostelium beta 1/2 could resemble a common ancestor of the more specialized beta 1 and beta 2 subunits of the vertebrate AP complexes. Our results support the essential contribution of a single beta subunit to the stability and function of AP1 and AP2 in a simple eukaryote.