We recently cloned and characterized a new putative λ subunit for voltage-gated calcium channels called λ7. This stargazin-like protein exhibits 25 % homology to λ2 (Moss et al. 2002). N-type current through Ca_V2.2 is markedly decreased when it is transiently co-expressed with λ7 in tsA-201 cells from -60.2 ± 10.7 pA pF^-1 (mean ± S.E.M., n = 44) to -15.1 ± 2.9 pA pF^-1 (n = 21) at +5 mV (Student’s unpaired t test, P < 0.01) (Fig. 1A). This has also been shown in COS-7 and Xenopus oocytes (Moss et al. 2002). Furthermore, this inhibition is not due to an interference with normal trafficking of Ca_V subunits but to a decrease of the level of expression of Ca_V2.2 protein (Fig. 1B).

Over-expression of λ7 in sympathetic neurons does not affect native N-type currents, suggesting that it does not alter pre-existing functional calcium channels (Moss et al. 2002). Inhibition requires translation of λ7 as N-type currents are not significantly reduced (-49.6 ± 8.6 pA pF^-1 at +5 mV, n = 28) when a stop codon is introduced two amino acids after the start codon of λ7. It was hypothesised that the last four amino acids of λ7 present in the C-terminus may be similar to the PDZ binding motif found to be important in other λ subunits. However, the removal of these four amino acids does not prevent the inhibition. Co-expression with this truncated λ7 resulted in a reduction to 21.1 ± 9.5 % (mean ± S.E.M., n = 16) of the control currents in Xenopus oocytes. We have also examined the selectivity of the inhibition by determining the effect of λ7 on other calcium channels and on a potassium channel. There was no effect of λ7 on either Ba²⁺ currents induced by expression of Ca_V3.1 (-9.8 ± 19.35 % inhibition at -10 mV, n = 17) or K⁺ currents induced by expression of K_V3.1b (11.3 ± 11.6 % inhibition at +60 mV, n = 9) when recorded in tsA-201 cells. It appears that λ7 inhibits the expression of high voltage activated calcium channels, particularly Ca_V2.2, at an early stage in synthesis. Inhibition requires translation of λ7 but is not dependent on the last four amino acids, hypothesised to be similar to a PDZ binding motif. Work is currently ongoing to elucidate the mechanism of inhibition.

This work was supported by the Wellcome Trust and BBSRC.