THE TOBACCO HOMOLOG OF MAMMALIAN CALRETICULIN PRESENT IN PROTEIN COMPLEXES IN-VIVO

The analysis of protein sorting signals responsible for the retention of reticuloplasmins (RPLs), a group of soluble proteins that reside in the lumen of the endoplasmic reticulum (ER), has revealed a structura l similarity between mammalian and plant ER retention signals. We pres ent evidence that the corresponding epitope is conserved in a vast fam ily of soluble ER resident proteins. Microsequences of RPL60 and RPL90 , two abundant members of this family, show high sequence similarity w ith mammalian calreticulin and endoplasmin. RPL60/calreticulin cofract ionates and costains with the lumenal binding protein (BiP). Both prot ei ns were detected in the nuclear envelope and the ER, and in mitotic cells in association with the spindle apparatus and the phragmoplast. Immunoprecipitation of proteins from in vivo-labeled cells demonstrat ed that RPL60/calreticulin is associated with other polypeptides in a stress- and ATP-dependent fashion. RPL60/calreticulin transcript level s increased rapidly in abundance during the proliferation of the secre tory apparatus and the onset of hydrolase secretion in gibberellic aci d-treated barley aleurone cells. This induction profile is identical t o that of the well-characterized ER chaperones BiP and endoplasmin. Ho wever, expression patterns in response to different stress conditions as well as tissue-specific expression patterns indicate that these gen es are differentially regulated and may not act in concert.