A MICROSOMAL ATP-BINDING PROTEIN INVOLVED IN EFFICIENT PROTEIN-TRANSPORT INTO THE MAMMALIAN ENDOPLASMIC-RETICULUM

Protein transport into the mammalian endoplasmic reticulum depends on nucleoside triphosphates. Photoaffinity labelling of microsomes with a zido-ATP prevents protein transport at the level of association of pre cursor proteins with the components of the transport machinery, Sec61 alpha and TRAM proteins. The same phenotype of inactivation was observ ed after depleting a microsomal detergent extract of ATP-binding prote ins by passage through ATP-agarose and subsequent reconstitution of th e pass-through into proteoliposomes, Transport was restored by co-reco nstitution of the ATP eluate, This eluate showed eight distinct bands in SDS gels. We identified five lumenal proteins (Grp170, Grp94, BiP/G rp78, calreticulin and protein disulfide isomerase), one membrane prot ein (ribophorin I) and two ribosomal proteins (L4 and L5), In addition to BiP (Grp78), Grp170 was most efficiently retained on ATP-agarose. Purified BiP did not stimulate transport activity, Sequence analysis r evealed a striking similarity of Grp170 and the yeast microsomal prote in Lhs1p which was recently shown to be involved in protein transport into yeast microsomes. We suggest that Grp170 mediates efficient inser tion of polypeptides into the microsomal membrane at the expense of nu cleoside triphosphates.