The gap junction gene connexin 43 (Cx43) showed tumor-suppressing effects on various tumor cell lines. We have previously demonstrated that Cx43 inhibited expression of S phase kinase-associated protein 2 (Skp2), the human F-box protein that regulates the ubiquitination of p27. Cx43 did not alter the mRNA level of SKP2, but it promoted the degradation of the Skp2 proteins (Zhang, Y. W., Nakayama, K., Nakayama K. I., and Morita, I. (2003) Cancer Res. 63, 1623-1630). In this study, we showed that the specific gap junction inhibitor 18 beta-glycyrrhetinic acid did not influence the inhibitory effect of Cx43 on Skp2 expression. Further, the deletion mutation analyses demonstrated that the C-terminal domain of Cx43 that did not form gap junctions was sufficient to inhibit expression of Skp2, whereas the N-terminal domain that formed the gap junctions did not show such an effect. Like the full-length Cx43, the C-terminal domain also increased the protein instability of Skp2, whereas the N terminus did not. Moreover, the C-terminal domain was as effective as the full-length Cx43 in inhibiting cell proliferation; however, the N-terminal domain did not show any inhibitory effect on cell proliferation. Therefore, these data revealed a gap junction-independent pathway for Cx43 to inhibit tumor growth by suppressing the Skp2 expression.