Effect of cell trypsinization on nuclear proteins of WI‐38 fibroblasts in culture

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Abstract
When resting confluent monolayers of WI‐38 fibroblasts are trypsinized and replated at a lower density they are stimulated to proliferate again with an interval of 18 hours between replating and the onset of DNA synthesis. Trypsinization of resting cells causes a 40% loss of nuclear proteins as well as of cytoplasmic proteins. The amount of nuclear proteins remains low for the first six hours after the cells have been replated and then it increases rapidly, reaching the same level of non‐trypsinized resting cells by ten hours after plating. The proteins that are lost from the nucleus immediately after trypsinization are chromatin‐associated proteins and most of them are non‐histone chromosomal proteins, although a modest loss of histones cannot be ruled out. The loss of non‐histone chromosomal proteins from cells that have been trypsinized causes changes in the structure of chromatin that can be detected by circular dichroism and by viscosity measurements. These results show that cell trypsinization causes an extensive loss of proteins from chromatin and that the loss is restored only several hours after the cells have been replated at a lower density.