The nascent polypeptide-associated complex (NAC) of yeast functions in thetargeting process of ribosomes to the ER membrane

We study here the binding of ribosomes to the endoplasmic reticulum (ER) me mbrane and its dependence on nascent polypeptide-associated complex (NAC), For this, we use an in vitro translation system in combination with isolate d microsomes, Importantly, all components in the system are derived from a single source, Saccharomyces cerevisiae. Ribosome nascent chains (RNCs) of the two naturally occurring invertase species (secreted or cytosolic) were prepared in wild-type, Delta alpha NAC or Delta alpha beta(1)beta(3)NAC tra nslation lysates and tested for binding to the corresponding microsomal mem branes, We provide evidence that NAC prevents binding of RNCs without a sig nal sequence to yeast membranes, In the absence of NAC, signal-less RNCs ar e able to bind to ER membranes. However, following puromycin treatment, onl y very few nascent chains translocate into the lumen, as detected by glycos ylation. (C) 1999 Federation of European Biochemical Societies.