Analysis of the oligomeric structure of the motor protein prestin.

Prestin, a member of the solute carrier family 26, is expressed in the basolateral membrane of outer hair cells. This protein provides the molecular basis for outer hair cell somatic electromotility, which is crucial for the frequency selectivity and sensitivity of mammalian hearing. It has long been known that there are abundantly expressed approximately 11-nM protein particles present in the basolateral membrane. These particles were hypothesized to be the motor proteins that drive electromotility. Because the calculated size of a prestin monomer is too small to form an approximately 11-nM particle, the possibility of prestin oligomerization was examined. We investigated possible quaternary structures of prestin by lithium dodecyl sulfate-PAGE, perfluoro-octanoate-PAGE, a membrane-based yeast two-hybrid system, and chemical cross-linking experiments. Prestin, obtained from different host or native cells, is resistant to dissociation by lithium dodecyl sulfate and behaves as a stable oligomer on lithium dodecyl sulfate-PAGE. In the membrane-based yeast two-hybrid system, homo-oligomeric interactions between prestin-bait/prestin-prey suggest that prestin molecules can associate with each other. Chemical cross-linking experiments, perfluoro-octanoate-PAGE/Western blot, and affinity purification experiments all indicate that prestin exists as a higher order oligomer, such as a tetramer, in prestin-expressing yeast, mammalian cell lines and native outer hair cells. Our data from experiments using hydrophobic and hydrophilic reducing reagents suggest that the prestin dimer is connected by a disulfide bond embedded in the prestin hydrophobic core. This stable dimer may act as the building block for producing the higher order oligomers that form the approximately 11-nM particles in the outer hair cell's basolateral membrane.

Effects of cyclic nucleotides on the function of prestin.

Outer hair cells (OHCs) in the mammalian organ of Corti display electromotility, which is thought to provide the local active mechanical amplification of the cochlear response. Prestin is the key molecule responsible for OHC electromotility. Several compounds, including cGMP, have been shown to influence OHC electromotility. There are two potential cAMP/cGMP-dependent protein kinase phosphorylation sites on prestin. Whether these sites are involved in cGMP-dependent reactions is as yet unknown. In this study, prestin cDNA was transiently transfected into TSA 201 cells. Cells that expressed prestin were selected to measure non-linear capacitance (NLC), a signature of outer hair cell motility. We applied cGMP and cAMP analogues and a protein kinase G (PKG) antagonist to the cells. Furthermore, nine mutations at putative phosphorylation sites of prestin were produced. The neutral amino acid alanine replaced serine/threonine at phosphorylation sites to change the conserved phosphorylation motif in order to mimic the dephosphorylated state of prestin, whereas replacement with the negatively charged aspartic acid mimicked the phosphorylated state. The properties of such modified prestin-expressing cells were examined, through measurement of NLC and with confocal microscopy. Our data demonstrate that cGMP is significantly more influential than cAMP in modifying the non-linear, voltage-dependent charge displacement in prestin-transfected cells. The electrical properties of the single and double mutations further indicate a possible interaction between the two PKG target sites. One of these sites may influence the membrane targeting process of prestin. Finally, a new topology map of prestin is proposed.

The C-terminus of prestin influences nonlinear capacitance and plasma membrane targeting.

Prestin is a unique molecular-motor protein expressed in the lateral plasma membrane of outer hair cells (OHC) in the organ of Corti of the mammalian cochlea. It is thought that prestin undergoes conformational changes driven by the cell's membrane potential. The resulting alterations in OHC-length are assumed to constitute the cochlear amplifier. Prestin is a member of the anion solute carrier family 26 (SCL26A), but it is different from other family members in its unique function of voltage-driven motility. Because the C-terminus is the least conserved region in the family, we investigated its influence with a series of deletion, point and chimeric mutants. The function and cellular expression of mutants were examined in a heterologous expression system by measurement of nonlinear capacitance (NLC) and immunofluorescence. Each mutant produced a unique mixture of patterns of cell morphologies, which were classified as to the location of prestin within the cell. The data from deletion mutants (Del516, Del525, Del630, Del590, Del709, Del719) revealed that nearly the full length (>708 amino acids) of the protein was required for normal prestin expression and function. Since most deletion mutations eliminated plasma membrane targeting, chimeric proteins were constructed by fusing prestin, at amino acid 515 or 644, with the homologous portion of the C-terminus from the two most closely related SLC26A members, pendrin and putative anion exchanger 1. These chimeric proteins were again improperly (but differently) targeted than simple truncation mutants, and all lacked functional phenotype. When two of the potential basolateral membrane-targeting motifs were mutated (Y520A/Y526A), incomplete plasma membrane expression was seen. We also show that some double point mutations (V499G/Y501H) fully express in the plasma membrane but lack NLC. These non-charged amino acids may have unrevealed important roles in prestin's function. Together, these data suggest that certain specific sequences and individual amino acids in the C-terminus are necessary for correct cellular distribution and function.