The assembly of the major desmosome glycoproteins of Madin-Darby canine kidney cells

Madin-Darby canine kidney (MDCK) cells are unable to form desmosomes when cultured in low-calcium medium ([Ca 2+]<0.1 meq./l), but can be induced to do so by raising the calcium to physiological concentrations (1–2 meq./l). We have previously demonstrated that this block correlated with increased desmosomal protein turnover. Here we have immunoprecipitated the major desmosome glycoproteins [DGI (150 kDa) and DGII/III (120/100 kDa)] from non-ionic detergent-soluble and -insoluble fractions prepared from metabolically labelled MDCK cells cultured in standard or low-calcium medium. Pulse-chase studies showed that both DGI and DGII/III became unextractable in non-ionic detergent before their arrival at the cell surface, whether cells were grown in standard or low-calcium medium. The non-ionic detergent insolubility of these membrane components is therefore a separate step which precedes the formation of morphologically recognisable desmosomes.