Skin tissue lysate was prepared by homogenization in lysis buffer (10 mM HEPES pH7.9, 1.5 mM MgCl2, 10 mM KCl, 1 mM ethylenediaminetetraacetic acid, 10% glycerol, 1% NP-40, and a cocktail of protease inhibitors). Tissue and cell debris was removed by centrifugation. The product was boiled for 5 min in 1 x SDS sample buffer (50 mM Tris-HCl pH 6.8, 12.5% glycerol, 1% sodium dodecylsulfate, 0.01% bromophenol blue) containing 5% beta-mercaptoethanol.