Solution properties of full length and truncated forms of myosin subfragment 1 from Dictyostelium discoideum.

The atomic structures for several myosin head isoforms in different nucleotide states have been determined in recent years. The comparison of these structures is complicated by the use of myosin subfragment 1 (S1) constructs of different length in different studies. Several atomic structures of the S1 nucleotide complex were obtained using Dictyostelium discoideum S1dC, a genetically truncated form of S1 lacking the light chain binding domain (LCBD) and both light chains. The goal of the present study has been to assess the effects of such a truncation on the solution properties of S1 and in particular, on its active site, actin binding site and the converter region. The nucleotide and actin binding properties, CD spectra and the reactivities of Lys-84 (corresponds to the 'reactive lysine', Lys-83 in rabbit skeletal S1) and Cys-678 (corresponds to the 'SH2-group', Cys-697 in rabbit S1) were compared for the full length (flS1) and the truncated (S1dC) forms of Dictyostelium S1. The two forms showed similar nucleotide binding properties. However, SldC had a lower structural stability and a significantly higher Km value for actin-activated ATPase as compared to flS1. Differences were found also in the near-UV CD spectrum between flS1 and S1dC. SH2 reactivity in SldC appeared to be greatly inhibited compared with that in flS1. The modification of Lys-84 caused a greater increase in the MgATPase activity in S1dC than in flS1. ADP inhibited this activation for both SldC and flS1. Taken together our results identify both truncation-caused differences between S1dC and flS1, as well as isoform-related differences between skeletal and Dictyostelium S1.