Hyaluronate appears to be covalently linked to the cell surface.

Abstract

The purpose of this study was to examine the nature of the linkage between cell-surface hyaluronate and the plasma membrane. To accomplish this, rat fibrosarcoma cells were cultured in the presence of [3H]-acetate to isotopically label the hyaluronate, and then fixed with glutaraldehyde, which cross-links proteins but does not react directly with hyaluronate. The glutaraldehyde fixation stabilized the cells so that they could be manipulated in ways which would otherwise destroy cells. The fixed cells were then subjected to various treatments, and the amount of hyaluronate remaining on the cell surface was assayed via exhaustive digestion with Streptomyces hyaluronidase. Using this technique, we found that 1) cell-surface hyaluronate was quite stable for extended periods of time even in the presence of a large excess of non-labeled hyaluronate; 2) 4 M guanidine HCl and detergents did not extract a significant portion of cell-surface hyaluronate; 3) solutions of varying ionic strength (0-1 M NaCl) had no effect on the retention of hyaluronate; 4) the cell coat was stable in the range of pH 4-11, but outside this range a significant amount of hyaluronate was released; and 5) treatment with proteases released cell-surface hyaluronate. These results are consistent with the possibility that hyaluronate is covalently linked to a protein associated with the plasma membrane. Further support for this model came from experiments with the detergent Triton X-114, which can be used to separate soluble proteins from hydrophobic proteins. When nonfixed rat fibrosarcoma cells were extracted with this detergent and then partitioned by centrifugation, approximately 30 times as much hyaluronate was present in the detergent fraction which contained the hydrophobic proteins, as compared to the extracts pretreated with trypsin prior to phase separation. Again, these results suggest that cell-surface hyaluronate is directly linked to a hydrophobic core protein intercalated in the plasma membrane.