Expression of Basement Membrane Proteins: Evidence for Complex Post-transcriptional Control Mechanisms

Abstract

Differentiated murine teratocarcinoma cell lines have been widely used as sources for the basement membrane proteins laminin and collagen IV. In order to understand the control of their expression, we have measured the transcription rates of the corresponding genes in nuclear run-on assays. The ratios of transcripts obtained from the five different genes of interest (for laminins A, B1, and B2 and α1(IV), α2(IV)) are rather different from the ratios of the corresponding mRNAs, which are again different from the protein levels needed. The gene for α2(IV) is transcribed at a higher rate than the one for α1(IV) and, similarly, the gene for laminin A is transcribed at a higher rate than the other two laminin genes, respectively. However, the α2(IV) and laminin A mRNA levels are lower than those for the other chains of the same molecule. The α1(IV) mRNA is 3- to 15-fold more abundant than the α2(IV) mRNA, depending on the cell line. At the protein level, the A chain seems to be limiting for the assembly of laminin, in accordance with its low mRNA level. The two collagen chains have variable pool sizes, but the triple helical molecules always seem to be composed of two α1(IV) and one α2(IV) chains. These results point to extensive control mechanisms at various stages of post-transcriptional events, some of which we could identify.