We performed a systematic mapping of interaction domains on COP I subunits to gain novel insights into the architecture of coatomer. Using the two‐hybrid system, we characterize the domain structure of the α‐, β′‐, ϵ‐COP and β‐, γ‐, δ‐, ζ‐COP coatomer subcomplexes and identify links between them that contribute to coatomer integrity. Our results demonstrate that the domain organization of the β‐, γ‐, δ‐, ζ‐COP subcomplex and AP adaptor complexes is related. Through in vivo analysis of α‐COP truncation mutants, we characterize distinct functional domains on α‐COP. Its N‐terminal WD40 domain is dispensable for yeast cell viability and overall coatomer function, but is required for KKXX‐dependent trafficking. The last∼ 170 amino acids of α‐COP are also non‐essential for cell viability, but required for ϵ‐COP incorporation into coatomer and maintainance of normal ϵ‐COP levels. Further, we demonstrate novel direct interactions of coatomer subunits with regulatory proteins: β′‐and γ‐COP interact with the ARF‐GTP‐activating protein (GAP) Glo3p, but not Gcs1p, and β‐and ϵ‐COP interact with ARF‐GTP. Glo3p also interacts with intact coatomer in vitro.