The isolation and identification of ubiquitin from the high mobility group (HMG) non-histone protein fraction.

Chromatin contains a group of non-histone chromosomal proteins called the high mobility group (HMG) proteins [l] . Four of the HMG proteins from calf thymus, HMG 1,2,14 and 17, and an HMG protein from trout testis, HMG-T, have been isolated in a pure form [2-51. The amino acid sequence of calf thymus HMG 17 has recently been determined [6] , and we have also shown that HMG proteins HMG 1,2, 14 and 17 are present in isolated nucleosomes [7]. Although we have shown that there are 4 main HMG components in thymus [3], we have consistently noted 3 weaker bands running faster than HMG 17 on polyacrylamide gels (see gels A and B of fig.1 in [3] and this paper). These proteins have been tentatively designated HMG 18, 19 and 20, in order of increasing electrophoretic mobility. We report in this paper the isolation of protein HMG 20 from calf thymus. Chemical analysis of this protein shows it to be the same as the isolated protein ubiquitin, a protein identified as part of a non-histone chromosomal protein complex, but the function of which is as yet unknown.