Cell biology aspects of ion channel functioning: microdomain formation, lateral mobility and ER export
Final Report Abstract
The plant K+ channel KAT1 is one of the most studied plant ion channels. In the past, studies of KAT1 have greatly increased our understanding of ion channel functioning in plants in general. In this project we have focused on cell biology aspects of channel functioning, namely the lateral distribution of KAT1 in the plasma membrane and its intracellular transport from the place of synthesis (endoplasmic reticulum) to the place of functioning (plasma membrane). KAT1 has previously been shown to be located in patches in the plasma membrane when expressed in plant or animal cells. Our investigation revealed that the patchy distribution of the channel is also found in yeast cells expressing KAT1. The KAT1 patches were unaffected by changes in membrane potential, sterol content or loss of the cell wall, when tested separately. Only in yeast mutants with reduced sterol content digestion of cell wall led to dispersion of KAT1. This implies that lateral segregation of KAT1 involves cooperate action of lipids (namely sterols) and cell wall components. Considering the stability of KAT1 patches, it may not be surprising that the channel shows virtually no lateral movement in the plasma membrane. In this project the low lateral mobility, which has previously been shown in plant cells, was extensively studied in animal cells expressing KAT1. Neither the sterol content nor the actin cytoskeleton was found to affect mobility of the channel. The stable patchy distribution and low lateral mobility of KAT1 in plant, mammalian and yeast cells implies that the mechanisms responsible for anchoring of KAT1 in the plasma membrane are highly conserved among eukaryotes. Trafficking of KAT1 from the endoplasmic reticulum (ER) to the plasma membrane has been shown to depend on ER export signals implying that ion channel density can be controlled at the site of ER export. Further analysis of the ER export signal carried out in this project revealed that the originally identified diacidic ER export motif is indeed a triacidic motif. Furthermore, function of this triacidic motif probably involves cooperative binding to Sec24, a component of the ER export machinery. Our investigations on the role of the ER export signal of KAT1 in targeting of channel heteromers demonstrated that the ER export signal of one channel subunit can affect targeting of other channels subunits. This in turn could lead to changes in the composition of channel heteromers in the plasma membrane. Considering that heteroerisation is found for many members of different K+ channel families, the impact of ER export motifs on targeting of channels subunits may well be of general importance for the control of ion channel composition and thus membrane conductance.
Publications
- (2008). Interaction of the K+ Channel KAT1 with the COPII component Sec24 depends on di-acidic ER-export motif. Plant J. 56: 997-1006
Sieben C, Mikosch M, Brandizzi F & Homann U
- (2009). ER export of KAT1 is correlated to the number of acidic residues within a triacidic motif. Traffic 10: 1481–1487
Mikosch M, Käberich K & Homann U
- (2009). How do ER export motifs work on ion channel trafficking? Current Opinion Plant Biology 6: 685-689
Mikosch M & Homann U
- (2010). Trafficking, lateral mobility and segregation of the plant K+ channel KAT1. Plant Biology 12: 99-104
Reuff M, Mikosch M & Homann U
