Calreticulin (CRT), calnexin (CLNX) and calmegin (CLMG) are members of a family of endoplasmic reticulum (ER) chaperones which fold newly synthesized polypeptides destined to other organelles or the plasma membrane. Aside from their role of foldases in the ER, all members of this family of proteins modulate Ca2+ uptake. In Xenopus oocytes and other cells, stimulation by G-protein and tyrosine coupled receptors results in ER by Ca2+ ATPases of the SERCA family. CLNX and CLMG bind to the COOH terminus of the SERCA2b pump, inhibiting Ca2+ oscillations. Unlike CRT, which is entirely lumenal, CLNX and CLMG have a cytosolic domain that is phosphorylated by multiple kinases. Phosphorylation of a conserved residue in CLNX and CLMG serves as a regulatory switch that controls the interaction of the chaperones with SERCA2b, and as a consequence, Ca2+ uptake into the ER and Ca2+ oscillations are inhibited During protein folding, CRT and CLNX interact with modulation of SERCA2b thiol groups that face the ER lumen (Specific Aim #1). Phosphorylation of CLNX controls protein folding and Ca2+ homeostasis in the ER. Hypothesis will be tested to advance our knowledge of the kinases and phosphatases that are active in the cytosolic domain of CLNX (Specific Aim #2) Immature Xenopus oocytes exhibit Ca2+ oscillations. However when they mature, the spatio-temporal patterns of Ca2+ release change from oscillatory to a single tidal wave. Hypotheses regarding the involvement of CLNX in determining the characteristics of Ca2+ signaling during oocyte maturation will be tested (Specific Aim 3). During conditions that induce ER stress, malfolded proteins accumulate in the ER. Cellular responses to cope with this stress include an immediate inhibition protein translation and a transcriptional up-regulation of ER chaperones. Amongst the proteins that are up-regulated are CRT and SERCA2b. Hypotheses regarding the involvement of CRT and CLNX in the control of ERCa2+ imaging techniques will be used. phosphorylation-mediated control of the interaction of this family of chaperones with SERCA2b is of significance since it suggests a bi-directional mode of communication between the Ca2+ signaling system and the folding machinery in the ER.
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