Identification of cDNA clones encoding different domains of the basement membrane heparan sulfate proteoglycan

Structure and biological activity of basement membrane proteins

Collagen type IV, laminin, heparan sulfate proteoglycans, nidogen (entactin) and BM-40 (osteonectin, SPARC) represent major structural proteins of basement membranes. They are well-characterized in their domain structures, amino acid sequences and potentials for molecular interactions. Such interactions include self-assembly processes and heterotypic binding between individual constituents, as well as binding of calcium (laminin, BM-40) and are likely to be used for basement membrane assembly. Laminin, collagen IV and nidogen also possess several cell-binding sites which interact with distinct cellular receptors. Some evidence exists that those interactions are involved in the control of cell behaviour. These observations have provided a more defined understanding of basement membrane function and the definition of new research goals in the future.

Identification of the precursor protein for the heparan sulfate proteoglycan of human colon carcinoma cells and its post-translational modifications

Human colon carcinoma cells synthesize a high-molecular-weight heparan sulfate proteoglycan which is localized at the cell surface. In this study we have performed a series of immunoprecipitation and pulse-chase experiments associated with various pharmacological agents that interfere with the synthesis and post-translational modification of the proteoglycan. We demonstrate that colon carcinoma cells synthesize the heparan sulfate proteoglycan from a 400-kDa precursor protein that is immunologically related to the Engelbreth-Holm-Swarm (EHS) tumor cell proteoglycan. The cells contain a large pool of precursor protein with a half-life of about 75 min. Most of the precursor protein receives heparan sulfate side chains and is then transported to the cell surface and released into the medium. A portion of the precursor pool, however, does not receive heparan sulfate chains but is secreted into the medium. The glycosylation and subsequent secretion of the 400-kDa precursor protein was inhibited by NH4Cl and even more by monensin, indicating that the transit of precursor from the rough endoplasmic reticulum to the cell surface occurred through the Golgi complex and acidic compartments. The existence of a sizable pool of precursor protein was confirmed by additional experiments using cycloheximide and xyloside. These experiments showed that the half-life of the precursor protein was also 75 min and that stimulation of heparan sulfate synthesis by xyloside was greatly enhanced (about 12-fold) after new protein core synthesis was blocked by cycloheximide. Although the structural models proposed for the EHS and colon carcinoma heparan sulfate proteoglycans differ, the observation that they are derived from a precursor protein with dimensional and immunological similarities suggests that they may be genetically related.