Density Measurements of Poly(Acrylic Acid) Potassium Salts

Density measurements of poly(acrylic acid) potassium salts (PAK) with different degrees of neutralization and water contents are presentea. The apparent partial molar volumes of polymer, V2, and the partial molar volumes of water, V1, were calculated from the densities. The values of V2 decreased with increasing water content and eventually leveled off. The values of V1, which at low water contents were much smaller than that of free water, increased with increasing water content and reached that of free water, showing consequently the appearance of free water. Before reaching the final value of free water, the data indicated the formation of primary and secondary hydration shells. The structure of primary, hydration was suggested to be of body-centered cubic coordination in which carboxyl oxygen atoms participate

A Rac GTPase-Activating Protein, MgcRacGAP, Is a Nuclear Localizing Signal-Containing Nuclear Chaperone in the Activation of STAT Transcription Factors▿ †

In addition to their pleiotropic functions under physiological conditions, transcription factors STAT3 and STAT5 also have oncogenic activities, but how activated STATs are transported to the nucleus has not been fully understood. Here we show that an MgcRacGAP mutant lacking its nuclear localizing signal (NLS) blocks nuclear translocation of p-STATs both in vitro and in vivo. Unlike wild-type MgcRacGAP, this mutant did not promote complex formation of phosphorylated STATs (p-STATs) with importin α in the presence of GTP-bound Rac1, suggesting that MgcRacGAP functions as an NLS-containing nuclear chaperone. We also demonstrate that mutants of STATs lacking the MgcRacGAP binding site (the strand βb) are hardly tyrosine phosphorylated after cytokine stimulation. Intriguingly, mutants harboring small deletions in the C′-adjacent region (βb-βc loop region) of the strand βb became constitutively active with the enhanced binding to MgcRacGAP. The molecular basis of this phenomenon will be discussed, based on the computer-assisted tertiary structure models of STAT3. Thus, MgcRacGAP functions as both a critical mediator of STAT's tyrosine phosphorylation and an NLS-containing nuclear chaperone of p-STATs

Solution structure of the antifreeze-like domain of human sialic acid synthase

The structure of the C-terminal antifreeze-like (AFL) domain of human sialic acid synthase was determined by NMR spectroscopy. The structure comprises one α- and two single-turn 310-helices and two β-strands, and is similar to those of the type III antifreeze proteins. Evolutionary trace analyses of the type III antifreeze protein family suggested that the class-specific residues in the human and bacterial AFL domains are important for their substrate binding, while the class-specific residues of the fish antifreeze proteins are gathered on the ice-binding surface