THE BINDING-PROTEIN ASSOCIATES WITH MONOMERIC PHASEOLIN

The association of the binding protein (BiP) with newly synthesized pr oteins in the endoplasmic reticulum (ER) of developing bean (Phaseolus vulgaris) cotyledonary cells was investigated. ATP-sensitive associat ion with many polypeptides was detected. The fraction of newly synthes ized polypeptides associated with BiP varies among different proteins. The relationship between subunit assembly and binding to BiP was inve stigated in the case of the vacuolar trimeric glycoprotein phaseolin. In spite of the presence of a significant pool of phaseolin trimers in the ER, only monomeric phaseolin is found in association with BiP. On the whole, our results point to a general role of BiP in the synthesi s of plant secretory proteins and indicate that, in the case of phaseo lin, BiP binding sites are concealed during structural maturation in t he ER, either before or upon formation of trimers. Our results also in dicate that trimerization does not constitute a rate-limiting step in the transport of phaseolin to the protein storage vacuoles.