Abstract
A. thaliana has 35 aquaporin species. They are subdivided into four families, PIP, TIP, NIP, and SIP. In this study, we analyzed SIPs which are not well known. When SIPs fused with green fluorescent protein were transiently expressed in protoplasts of A. thaliana cultured cells, green fluorescence was observed specifically in the ER. Furthermore, SIPs were immunochemically detected in the ER fraction after sucrose density gradient centrifugation. Then SIPs were expressed in yeast and the water permeability of membranes from the yeast transformants was assayed by stopped-flow spectrophotometry. Three SIPs had little water channel activity. The SIP promoter region of 2 kb has been fused to a GUS reporter gene and expressed in A. thaliana. The GUS activity was detected in a tissue-specific manner. Thus, SIPs may function as slow water channel or the transporter for other small molecules at the ER membrane and maintain the ER homeostasis.
