Mouse SAA(1) and SAA(2) bind avidly to heat denatured ubiquitin

Ubiquitin (UB) covalently linked to abnormal proteins acts a signal for attack by proteinases; non-covalent binding of UB to cellular proteins has also been predicted. Here we show non-covalent binding of heat-denatured UB to an acute phase protein, serum amyloid A (SAA). Four solid-phase binding assays were used to determine UB-SAA interaction. Mouse SAA rich plasma was incubated with UB affinity matrix; SAA in the eluted proteins was identified by specific antibodies and N-terminal sequence analysis. The results show that both murine SAA(1) and SAA(2) isotypes bind avidly and selectively to UB indicating a role for UB in SAA processing.