Crucial role of N terminus in function of cardiac L-type Ca super(2+) channel and its modulation by protein kinase C

• Published in: The Journal of biological chemistry 1998-07, Vol.273 (28), p.17901-17909
• Main Authors: Shistik, E, Ivanina, T, Blumenstein, Y, Dascal, N
• Format: Article
• Language: English
• ISSN: 0021-9258

The role of the cytosolic N terminus of the main subunit ( alpha sub(1C)) of cardiac L-type voltage-dependent Ca super(2+) channel was studied in Xenopus oocyte expression system. Deletion of the initial 46 or 139 amino acids (a.a.) of rabbit heart alpha sub(1C) caused a 5-10-fold increase in the whole cell Ca super(2+) channel current carried by Ba super(2+) (I sub(Ba)), as reported previously. The plasma membrane content of alpha sub(1C) protein, measured immunochemically, was not altered by the 46-a.a. deletion. Patch clamp recordings in the presence of a dihydropyridine agonist showed that this deletion causes a similar to 10-fold increase in single channel open probability without changing channel density. Thus, the initial segment of the N terminus affects channel gating rather than expression. The increase in I sub(Ba) caused by coexpression of the auxiliary beta sub(2A) subunit was substantially stronger in channels with full-length alpha sub(1C) than in 46- or 139-a.a. truncated mutants, suggesting an interaction between beta sub(2A) and N terminus. However, only the I-II domain linker of alpha sub(1C), but not to N or C termini, bound beta sub(2A) in vitro. The well documented increase of I sub(Ba) caused by activation of protein kinase C (PKC) was fully eliminated by the 46-a.a. deletion. Thus, the N terminus of alpha sub(1C) plays a crucial role in channel gating and PKC modulation. We propose that PKC and beta subunit enhance the activity of the channel in part by relieving an inhibitory control exerted by the N terminus. Since PKC up-regulation of L-type Ca super(2+) channels has been reported in many species, we predict that isoforms of alpha sub(1C) subunits containing the initial N-terminal 46 a.a. similar to those of the rabbit heart alpha sub(1C) are widespread in cardiac and smooth muscle cells.