Paxillin, a focal adhesion protein, exists as multiple isoforms in humans (α, β, and γ). To understand more about the physiological role of each isoform, we have employed the mouse system. We found that although the α and β isoforms are present in the mouse, the γ isoform is not. The α isoform protein was detected clearly in most adult tissues, whereas the β isoform protein was almost undetectable except in spleen, testis, thymus, and lung. On the other hand, mRNAs of both isoforms were detectable in all tissues we examined. High levels of the β isoform protein was detected in peritoneal exudate macrophage cells in adult mouse as well as in cultured fibroblasts, together with the α isoform. The α isoform was expressed at a constant level throughout the embryonic stages we examined, whereas the β isoform protein was detected at the mid-stages of development and increased to levels almost equal to those of the α isoform during the late stages of embryogenesis. Therefore, unlike the α isoform, expression of the β isoform protein is restricted in adult tissues. Moreover, we showed that α and β isoforms were colocalized within the same focal adhesion plaques, and cytoplasmic pools of both isoforms exist in the perinuclear area, colocalized with the Golgi apparatus.
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