Inner membrane YfgM-PpiD heterodimer acts as a functional unit that associates with the SecY/E/G translocon and promotes protein translocation.

PpiD and YfgM are inner membrane proteins that are both composed of an N-terminal transmembrane segment and a C-terminal periplasmic domain. Escherichia coli YfgM and PpiD form a stable complex that interacts with the SecY/E/G (Sec) translocon, a channel that allows protein translocation across the cytoplasmic membrane. Although PpiD is known to function in protein translocation, the functional significance of PpiD-YfgM complex formation as well as the molecular mechanisms of PpiD-YfgM and PpiD/YfgM-Sec translocon interactions remain unclear. Here, we conducted genetic and biochemical studies using yfgM and ppiD mutants and demonstrated that a lack of YfgM caused partial PpiD degradation at its C-terminal region and hindered the membrane translocation of Vibrio protein export monitoring polypeptide (VemP), a Vibrio secretory protein, in both E. coli and Vibrio alginolyticus. While ppiD disruption also impaired VemP translocation, we found that the yfgM and ppiD double deletion exhibited no additive or synergistic effects. Together, these results strongly suggest that both PpiD and YfgM are required for efficient VemP translocation. Furthermore, our site-directed in vivo photocrosslinking analysis revealed that the tetratricopeptide repeat domain of YfgM and a conserved structural domain (NC domain) in PpiD interact with each other and that YfgM, like PpiD, directly interacts with the SecG translocon subunit. Crosslinking analysis also suggested that PpiD-YfgM complex formation is required for these proteins to interact with SecG. In summary, we propose that PpiD and YfgM form a functional unit that stimulates protein translocation by facilitating their proper interactions with the Sec translocon.

Nucleosomal histone kinase-1 phosphorylates H2A Thr 119 during mitosis in the early Drosophila embryo.

Posttranslational histone modifications are important for the regulation of many biological phenomena. Here, we show the purification and characterization of nucleosomal histone kinase-1 (NHK-1). NHK-1 has a high affinity for chromatin and phosphorylates a novel site, Thr 119, at the C terminus of H2A. Notably, NHK-1 specifically phosphorylates nucleosomal H2A, but not free H2A in solution. In Drosophila embryos, phosphorylated H2A Thr 119 is found in chromatin, but not in the soluble core histone pool. Immunostaining of NHK-1 revealed that it goes to chromatin during mitosis and is excluded from chromatin during S phase. Consistent with the shuttling of NHK-1 between chromatin and cytoplasm, H2A Thr 119 is phosphorylated during mitosis but not in S phase. These studies reveal that NHK-1-catalyzed phosphorylation of a conserved serine/threonine residue in H2A is a new component of the histone code that might be related to cell cycle progression.

Purification, properties, and partial amino acid sequences of alanine racemase from the muscle of the black tiger prawn Penaeus monodon.

Alanine racemase [EC 5.1.1.1], which catalyzes the interconversion between D- and L-alanine, was purified to homogeneity from the muscle of black tiger prawn Penaeus monodon. The isolated enzyme had a molecular mass of 44 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and 90 kDa on gel filtration, indicating a dimeric nature of the enzyme. The enzyme was highly specific to D- and L-alanine and did not catalyze the racemization of other amino acids. K(m) values toward both D- and L-alanine were almost equal and considerably high compared with those of bacterial enzymes. The purified enzyme retained its activity in the absence of pyridoxal 5'-phosphate as a cofactor but carbonyl reagents inhibited the activity, suggesting the tightly binding of the cofactor to the enzyme protein. Several partial amino acid sequences of peptide fragments of the purified enzyme showed positive homologies from 52 to 76% with bacterial counterparts and a catalytic tyrosine residue of the bacterial enzyme was also retained in the prawn one, indicating alanine racemase gene is well conserved from bacteria to invertebrates.