Clathrin-associated adaptor protein (AP) complexes are major structural com
ponents of clathrin-coated vesicles, functioning in clathrin coat assembly
and cargo selection. We have carried out a systematic biochemical and genet
ic characterization of AP complexes in Saccharomyces cerevisiae. Using coim
munoprecipitation, the subunit composition of two complexes, AP-1 and AP-2R
, has been defined. These results allow assignment of the 13 potential AP s
ubunits encoded in the yeast genome to three AP complexes. As assessed by i
n vitro binding assays and coimmunoprecipitation, only AP-1 interacts with
clathrin. Individual or combined disruption of AP-1 subunit genes in cells
expressing a temperature-sensitive clathrin heavy chain results in accentua
ted growth and cy-factor pheromone maturation defects, providing further ev
idence that AP-1 is a clathrin adaptor complex. However, in cells expressin
g wild-type clathrin, the same AP subunit deletions have no effect on growt
h or ct-factor maturation. Furthermore, gel filtration chromatography revea
led normal elution patterns of clathrin-coated vesicles in cells lacking AP
-1. Similarly, combined deletion of genes encoding the beta subunits of the
three AP complexes did not produce defects in clathrin-dependent sorting i
n the endocytic and vacuolar pathways or alterations in gel filtration prof
iles of clathrin-coated vesicles. We conclude that AP complexes are dispens
able for clathrin function in S, cerevisiae under normal conditions. Our re
sults suggest that alternative factors assume key roles in stimulating clat
hrin coat assembly and cargo selection during clathrin-mediated vesicle for
mation in yeast.