Mechanisms of Transport by SLC4 Borate Transporters - Project Summary/Abstract
The interior and exterior of cells are delineated by a mostly impermeable barrier called the cell
membrane. Proteins called membrane transporters enable the transport of molecules across
this barrier to fulfill cellular needs such as the export of toxic substances or the import of
essential nutrients. Many human diseases are associated with defects in the transport
mechanisms of membrane transport proteins. The proposed research focuses on the SLC4
membrane transporter family, which includes a human protein called AE1 that is essential for
driving cellular respiration. AE1 is the most abundant membrane protein in red blood cells, and
modulates CO2 transport by exchanging bicarbonate and chloride ions. Diseases such as renal
tubular acidosis, hereditary spherocytosis, and hereditary stomatocytosis can be caused by
mutations in the membrane transport domain of AE1. We study yeast and plant homologs of
human AE1 because they provide experimental advantages such as the ability to test their
function through a simple but powerful genetic complementation assay. Our long-term goal is
to study these homologs to elucidate the molecular mechanism of SLC4 transport
activity and thus shed light on the molecular basis of these diseases and set the stage
for the development of treatments. Yeast and plants each have SLC4 proteins that transport
borate, which is chemically similar to bicarbonate. In yeast, the transport activity of the SLC4
protein Bor1 is to expel borate, which is toxic in excess. In plants, borate is an essential
micronutrient that must be imported into plants to support their cell walls, but exported when in
toxic levels of excess. The primary goal of the proposed research is to dissect the mechanism
of transport in SLC4 borate transporters. We will do so through a combination of mutagenesis
and functional assays, informed by our knowledge of structures of Bor1 and AE1. These
experiments will identify and map amino acids that play functional roles in transport, determine
how and whether the quaternary structure of SLC4 proteins controls transport activity, and
dissect how the substrate binding affinity can explain distinct functional roles for different
borate transporters.
