REGULATION OF SYNTHESIS AND TURNOVER OF MAIZE AUXIN-BINDING PROTEIN AND OBSERVATIONS ON ITS PASSAGE TO THE PLASMA-MEMBRANE - COMPARISONS TOMAIZE IMMUNOGLOBULIN-BINDING PROTEIN COGNATE

Electrophysiological experiments have indicated that a fraction of the major auxin-binding protein (ABP1) of maize (Zea mays L.) Fight be a receptor on the outer surface of the plasma membrane. The predominant location of ABP1 is in the lumen of the endoplasmic reticulum (ER), in accord with its C-terminal KDEL retention signal. Little is known abo ut the biology of the protein in vivo or the rate at which it might pa ss to the cell surface. We have examined the turnover of ABP1 by in vi vo labelling of maize coleoptile sections. After different chase times , ABP1 was immunoprecipitated from detergent-solubilised membrane prep arations. Two polypeptides coprecipitated with ABP1. Neither was recog nised by any ABP1 antibodies nor by monoclonals to ER retention sequen ces. The possible significance of these coprecipitating polypeptides i s discussed. In addition, we have used a monoclonal antibody to precip itate HDEL proteins from the same membrane preparations. Two dimension al electrophoresis and N-terminal sequencing showed that the major HDE L protein precipitated was a member of the heat-shock-protein 70 famil y, a homologue of BiP (immunoglobulin-binding protein). We have invest igated the turnover of this BiP homologue for comparison with ABP1 and found that both had extended lifetimes, with half-lives greater than 24 h. Use of cordycepin to inhibit transcription indicated that ABP1 m RNA was also long-lived. Synthesis of ABP1 was strongly reduced by hea t stress, was reduced a little in response to dithiothreitol and was n ot markedly changed by tunicamycin. In contrast, BiP synthesis increas ed markedly in response to tunicamycin and dithiothreitol and increase d a little after heat stress. Neither auxin nor other plant growth reg ulators altered ABP1 synthesis or turnover. Auxin binding is thought t o induce a conformational change in ABP1 which might be associated wit h its escape from the ER. Incubation of coleoptile sections in auxin d id not alter the balance of ABP1 between ER-enriched and plasmamembran e-enriched fractions over prolonged chase times. We discuss the turnov er characteristics of ABP1 in terms of its known cellular distribution and of its function.