Dilysine motifs in cytoplasmic domains of transmembrane proteins are s
ignals for their continuous retrieval from the Golgi back to the endop
lasmic reticulum (ER). We describe a system to assess retrieval to the
ER in yeast cells making use of a dilysine-tagged Ste2 protein, Where
as retrieval was unaffected in most sec mutants tested (sec7, sec12, s
ec13, sec16, sec17, sec18, sec19, sec22, and sec23), a defect in retri
eval was observed in previously characterized coatomer mutants (sec21-
1, sec27-1), as well as in newly isolated retrieval mutants (sec21-2,
ret1-1). RET1 was cloned by complementation and found to encode the al
pha subunit of coatomer. While temperature-sensitive for growth, the n
ewly isolated coatomer mutants exhibited a very modest defect in secre
tion at the nonpermissive temperature. Coatomer from beta'-COP (sec27-
1) and alpha-COP (ret1-1) mutants, but not from gamma-COP (sec21) muta
nts, had lost the ability to bind dilysine motifs in vitro. Together,
these results suggest that coatomer plays an essential role in retrogr
ade Golgi-to-ER transport and retrieval of dilysine-tagged proteins ba
ck to the ER.