Two new low-molecular-weight acidic proteins from calf thymus nuclei that resemble HMG (high-mobility-group) proteins 14 and 17.

Two new, closely similar, acidic proteins were extracted and purified from calf thymus and designated AP-X and AP-Y (acidic proteins X and Y). They contain about 33% acidic residues, mostly non-amidated, and 20% lysine, but no arginine, tyrosine, histidine or tryptophan. There is a single phenylalanine residue in a molecular weight of approx. 5000. Circular dichroism and proton nuclear magnetic resonance show that they do not take up secondary or tertiary structure in free solution, as expected from the low content of hydrophobic amino acids. They appear structurally related to the high-mobility-group proteins HMG 14 and 17. Controlled extraction experiments indicate that proteins AP-X and AP-Y are at least partially located in the calf thymus nucleus.