1.A.33 The Cation Channel-forming Heat Shock Protein-70 (Hsp70) Family
The ubiquitous Hsp70 family includes molecular chaperones that are often found in association with membranes. One member of the Hsp70 family, Hsc70, has been incorporated into artificial lipid bilayers, and it proved to mediate stable transmembrane ion flow. Cation-selective flux occurred in well-defined, multilevel, discrete events suggesting the formation of a multi-conductance ion channel. Channel activity was ATP-dependent and ADP inhibitable (Arispe and DeMaio, 2000).
Cell surface localized Hsp70 binds a 32 kDa serine protease called granzyme B and takes up the protein. Uptake results in the initiation of apoptosis selectively in tumor cells presenting Hsp70 on their surfaces. The same Hsp70 could form cation-specific channels. Hsp70 may also promote cell lysis (Gross et al., 2003).
Eukaryotic members of the Hsp70 family include four well-characterized members: Hsp70, Hsc70, BIP and Mtp70. In all of these proteins, the N-terminal 44 kDa domain hydrolyzes ATP; the central 18 kDa domain interacts with target proteins (the peptide binding domain), and the C-terminal 10 kDa domain functions to allow association with co-chaperone proteins. Binding of ATP and co-chaperone influences peptide recognition while ATP hydrolysis increases the affinity for substrate peptides. Hsp70 and Hsc70 are cystolic, BIP is in the ER matrix, and Mtp70 is in the mitochondrial matrix. All of these proteins function in transmembrane polypeptide translocation. Whether channel formation is physiologically important in cells has not been determined.
The outer membranes of Mycobacteria form a thick permeability barrier and provide resistance to many antibiotics. Only few mycolate outer membrane (MOM) proteins have been identified. One identified MOMP, MMAR_0617 of Mycobacterium marinum ((B2HP37; TC# ), was purified and shown to form a large oligomeric complex with a clear single-channel conductance of 0.8 ± 0.1 ns upon reconstitution into artificial planar lipid bilayers. It has a long C-terminal threonine-rich domain with extensive modifications. It is distantly related to Hsp70 (van der Woude et al. 2013).
There is no evidence that the bacterial chaparone proteins of this family can form pores. These chaparone proteins are homologous to proteins involved in capsular polysaccharide export (TC# 9.A.41) and cell shape-determining complexes (TC# 9.B.157).
The transport reaction catalyzed by Hsc70 is:
ions (in) → ions (out)