α-Catenin is an essential component of the cadherin–catenin cell–cell adhesion complex. An excess amount of α-catenin also affects the Wnt signaling pathway probably through its direct binding to β-catenin. Here, we examined the molecular mechanisms of the posttranscriptional regulation of α-catenin expression. We constructed an expression vector with α-catenin cDNA lacking the 5′-untranslated sequence. In L cell transfectants stably expressing mRNA derived from this vector, the amount of exogenous α-catenin protein was about 10-fold higher than that of the endogenous protein. The expression level of the exogenously expressed α-catenin mRNA, however, was about 80% of that of endogenous molecule. Most of the endogenous and exogenous α-catenin protein in cadherin-negative cells was degraded 5 h after inhibition of protein synthesis. Although α-catenin contains the PEST sequence, various proteasome and calpain inhibitors did not affect the level of expression of endogenous α-catenin protein in L cells. Overexpressed α-catenin showed cytoplasmic localization, disturbed the nuclear localization of stabilized β-catenin, and inhibited TCF-4-responsive transactivation after Wnt-3a treatment. These results suggested that the low-efficiency of translation and unidentified degradation mechanisms maintained the low levels of α-catenin expression in the cytoplasm as a necessary condition for the Wnt signaling pathway.