Effects of An Acute Increase in Atrial Pressure on Atrial Refractoriness in Humans

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73199/1/j.1540-8159.1992.tb02954.x.pd

The Different Function of Single Phosphorylation Sites of Drosophila melanogaster Lamin Dm and Lamin C

Lamins' functions are regulated by phosphorylation at specific sites but our understanding of the role of such modifications is practically limited to the function of cdc 2 (cdk1) kinase sites in depolymerization of the nuclear lamina during mitosis. In our study we used Drosophila lamin Dm (B-type) to examine the function of particular phosphorylation sites using pseudophosphorylated mutants mimicking single phosphorylation at experimentally confirmed in vivo phosphosites (S25E, S45E, T435E, S595E). We also analyzed lamin C (A-type) and its mutant S37E representing the N-terminal cdc2 (mitotic) site as well as lamin Dm R64H mutant as a control, non-polymerizing lamin. In the polymerization assay we could observe different effects of N-terminal cdc2 site pseudophosphorylation on A- and B-type lamins: lamin Dm S45E mutant was insoluble, in contrast to lamin C S37E. Lamin Dm T435E (C-terminal cdc2 site) and R64H were soluble in vitro. We also confirmed that none of the single phosphorylation site modifications affected the chromatin binding of lamin Dm, in contrast to the lamin C N-terminal cdc2 site. In vivo, all lamin Dm mutants were incorporated efficiently into the nuclear lamina in transfected Drosophila S2 and HeLa cells, although significant amounts of S45E and T435E were also located in cytoplasm. When farnesylation incompetent mutants were expressed in HeLa cells, lamin Dm T435E was cytoplasmic and showed higher mobility in FRAP assay

Requirements for Efficient Proteolytic Cleavage of Prelamin A by ZMPSTE24

The proteolytic maturation of the nuclear protein lamin A by the zinc metalloprotease ZMPSTE24 is critical for human health. The lamin A precursor, prelamin A, undergoes a multi-step maturation process that includes CAAX processing (farnesylation, proteolysis and carboxylmethylation of the C-terminal CAAX motif), followed by ZMPSTE24-mediated cleavage of the last 15 amino acids, including the modified C-terminus. Failure to cleave the prelamin A "tail", due to mutations in either prelamin A or ZMPSTE24, results in a permanently prenylated form of prelamin A that underlies the premature aging disease Hutchinson-Gilford Progeria Syndrome (HGPS) and related progeroid disorders.Here we have investigated the features of the prelamin A substrate that are required for efficient cleavage by ZMPSTE24. We find that the C-terminal 41 amino acids of prelamin A contain sufficient context to allow cleavage of the tail by ZMPSTE24. We have identified several mutations in amino acids immediately surrounding the cleavage site (between Y646 and L647) that interfere with efficient cleavage of the prelamin A tail; these mutations include R644C, L648A and N650A, in addition to the previously reported L647R. Our data suggests that 9 of the 15 residues within the cleaved tail that lie immediately upstream of the CAAX motif are not critical for ZMPSTE24-mediated cleavage, as they can be replaced by the 9 amino acid HA epitope. However, duplication of the same 9 amino acids (to increase the distance between the prenyl group and the cleavage site) impairs the ability of ZMPSTE24 to cleave prelamin A.Our data reveals amino acid preferences flanking the ZMPSTE24 cleavage site of prelamin A and suggests that spacing from the farnesyl-cysteine to the cleavage site is important for optimal ZMPSTE24 cleavage. These studies begin to elucidate the substrate requirements of an enzyme activity critical to human health and longevity

The role of isoprenylation in membrane attachment of nuclear lamins : a single point mutation prevents proteolytic cleavage of the lamin A precursor and confers membrane binding properties

Mature A- and B-type lamins differ in the extent to which they interact with the nuclear membrane and thus represent an interesting model for studying the role of isoprenylation and carboxyl-methylation in membrane attachment. Both A- and B-type lamins are isoprenylated and carboxyl-methylated shortly after synthesis, but A-type lamins undergo a further proteolytic cleavage which results in the loss of the hydrophobically modified C terminus. Here, we have constructed mutants of chicken lamin A that differ in their abilities to serve as substrates for different post-translational processing events occurring at the C terminus of the wild-type precursor. In addition to studying full-length proteins, we have analyzed C-terminal end domains of lamin A, either alone or after fusion to reporter proteins. Mutant proteins were expressed in mammalian cells, and their membrane association was analyzed by immunofluorescence microscopy and subcellular fractionation. Our results provide information on the substrate specificity and subcellular localization of the lamin-A-specific protease. Moreover, they indicate that hydrophobic modifications of the C-terminal end domains account for the differential membrane-binding properties of A- and B-type lamins. Thus, some of the integral membrane proteins implicated in anchoring B-type lamins to the membrane may function as receptors for the isoprenylated and carboxyl-methylated C terminus

Phosphorylation on protein kinase C sites inhibits nuclear import of lamin B2

The nuclear lamina is a karyoskeletal structure at the nucleoplasmic surface of the inner nuclear membrane. Its assembly state is regulated by phosphorylation of the intermediate filament type lamin proteins. Strong evidence has been obtained for a causal link between phosphorylation of lamins by the p34cdc2 protein kinase and disassembly of the nuclear lamina during mitosis. In contrast, no information is currently available on the role of lamin phosphorylation during interphase of the cell cycle. Here, we have identified four protein kinase C phosphorylation sites in purified chicken lamin B2 as serines 400, 404, 410, and 411. In vivo, the tryptic peptide containing serines 400 and 404 is phosphorylated throughout interphase, whereas serines 410 and 411 become phosphorylated specifically in response to activation of protein kinase C by phorbol ester. Prompted by the close proximity of serines 410/411 to the nuclear localization signal of lamin B2, we have studied the influence of phosphorylation of these residues on nuclear transport. Using an in vitro assay, we show that phosphorylation of lamin B2 by protein kinase C strongly inhibits transport to the nucleus. Moreover, phorbol ester treatment of intact cells leads to a substantial reduction of the rate of nuclear import of newly synthesized lamin B2 in vivo. These findings have implications for the dynamic structure of the nuclear lamina, and they suggest that the modulation of nuclear transport rates by cytoplasmic phosphorylation may represent a general mechanism for regulating nuclear activities

Phosphorylation of Na,K-ATPase alpha-subunits in microsomes and in homogenates of Xenopus oocytes resulting from the stimulation of protein kinase A and protein kinase C.

The phosphorylation of the alpha-subunit of Na+/K(+)-transporting ATPase (Na,K-ATPase) by cAMP-dependent protein kinase (PKA) and protein kinase C (PKC) was characterized in purified enzyme preparations of Bufo marinus kidney and duck salt gland and in microsomes of Xenopus oocytes. In addition, we have examined cAMP and phorbol esters, which are stimulators of PKA and PKC, respectively, for their ability to provoke the phosphorylation of alpha-subunits of Na,K-ATPase in homogenates of Xenopus oocytes. In the enzyme from the duct salt gland, phosphorylation by PKA and PKC occurs on serine and threonine residues, whereas in the enzyme from B. marinus kidney and Xenopus oocytes, phosphorylation by PKA occurs only on serine residues. Phosphopeptide analysis indicates that a site phosphorylated by PKA resides in a 12-kDa fragment comprising the C terminus of the polypeptide. Studies of phosphorylation performed on homogenates of Xenopus oocytes show that not only endogenous oocyte Na,K-ATPase but also exogenous Xenopus Na,K-ATPase expressed in the oocyte by microinjection of cRNA can be phosphorylated in response to stimulation of oocyte PKA and PKC. In conclusion, these data are consistent with the possibility that the alpha-subunit of Na,K-ATPase can serve as a substrate for PKA and PKC in vivo