Ca2+-dependent and -independent mechanisms of calmodulin nuclear translocation

Abstract

Translocation from the cytosol to the nucleus is a major response by calmodulin (CaM) to stimulation of cells by Ca2+. However, the mechanisms involved in this process are still controversial and both passive and facilitated diffusion have been put forward. We tested nuclear translocation mechanisms in electroporated HeLa cells, rat cortical neurons and glial cells using novel calmodulin and inhibitor peptide probes and confocal microscopy. Passive diffusion of calmodulin across the nuclear membrane was measured in conditions in which facilitated transport was blocked and was compared to that of a similarly sized fluorescein-labeled dextran. Wheat germ agglutinin, which blocks facilitated transport but not passive diffusion, inhibited the nuclear entry of both wild-type and Ca2+-binding-deficient mutant calmodulin both in low and elevated [Ca2+]. Ca2+-dependent nuclear translocation was prevented by a membrane-permeant CaM inhibitor, the mTrp peptide, which indicated that it was specific to Ca2+/CaM. Diffusion of free CaM and Ca2+/CaM was considerably slower than the observed nuclear translocation by facilitated transport. Our data show that the majority of CaM nuclear entry occurred by facilitated mechanisms in all cell types examined, in part by a Ca2+-independent and in part by a Ca2+-dependent translocation mechanism.

S100A16, a Novel Calcium-binding Protein of the EF-hand Superfamily

“…Like other members of the family, S100A16 lacks the canonical nuclear localization signal. Nuclear import might then require interaction with transporter proteins as it was described for S100A11 (63) or might occur via alternative facilitated-diffusion pathways as observed for calmodulin (64). Furthermore, phosphorylation has been reported to be essential for S100A11 nuclear import (49,63).…”

The Calmodulin-binding Site of Sphingosine Kinase and Its Role in Agonist-dependent Translocation of Sphingosine Kinase 1 to the Plasma Membrane

“…Alternatively, it has been proposed that distinct pools of CaM exist in the cell that may exert their effects on different CaM targets depending on the location, duration, and magnitude of the intracellular Ca 2ϩ flux (19). Such a proposal may not only provide an explanation for the different effects of CaM on hSK1 and hSK2 but may also help to explain why CaM does not mediate redistribution of hSK1 to the nucleus, rather than the plasma membrane, despite the majority of CaM moving to the nucleus following increases in intracellular Ca 2ϩ (52). Clearly, further studies are required to understand these processes.…”

An update on nuclear calcium signalling

“…One such moiety is the ubiquitous Ca 2+ -binding protein calmodulin, which has been proposed to translocate into the nucleus following cellular stimulation. Although translocation of calmodulin from the cytosol into the nucleus has been observed in numerous cell types following elevation of cytosolic Ca 2+ (Mermelstein et al, 2001;Thorogate and Torok, 2004;Wu and Bers, 2007), the mechanism by which calmodulin enters the nucleus is not clear. Calmodulin does not contain an obvious motif for nuclear localisation, and might therefore 'piggyback' on other proteins that are translocated into the nucleus (Thorogate and Torok, 2007).…”