Introduction Sodium cotransport proteins Ernest M. Wright, Karl M. Hager and Eric Turk University of California at Los Angeles, Los Angeles, California, USA Significant advances have been made in elucidating the structure of Na+ cotransport proteins. Some fifteen of these low-abundance proteins have been cloned, sequenced and functionally expressed. They are members of the 12 membrane-spanning superfamily and they segregate into two groups, the Na+/glucose (SCLTI) and Na + /Cl-ICABA (CAT-I) families. SCLTI transporters are expressed in bacteria and animal cells, while CAT-1 transporters are mostly expressed in the brain. None have yet been found in plants. Current Opinion in Cell Biology 1992, 4:696-702 Since the cloning of the tirst euka~ote Na+ cotr;ms- porter [ 11, much progress has been made \\ith the cloning of other cotransporters. It is now e\-ident that there are at least wo unrelated gene families: one homologous to the intestinal and ren:d Na+ /glucose cotransporter ( SGLTl ), and another nith the brain Na+ ,/Cl-/GABA cotransporter (GAY1 1. The N:l+ cotrdns- porters belong to a clrlss of prok;t~0tic and eukar~.- otic membrane proteins, the secondaT active trans- porters (cotransporters and antiporters ). The cotrans- porters (siiporters 1 iire responsible for the active ac- cumulation of nutrients. neUrotT;inslllitters. vitamins, Me salts and ions within cells. The energ). to dri1.e the in- tracellular accumulation of these solutes is derived from the ion gradients (AL,, or A&+,) across the cell mem- brane, which are produced and maintained 131, primaq active transporters (H + and Naf J K+ pumps 1. In this re\ic? we summarize the recent progress with the Na + cotransporter proteins. The SCLTl family SCLTl The tirst m%unmali:tn cotransporters to he cloned lverc the Klbhit and human Na+ glucose cotransporters [ 1 .L], The amino acid sequence and :l seconda? structure model for the human SGLTl is shonn in Fig. I. The cDNA encodes a 664 residue protein that is predicted to cross the plasma membrane 12 times. Hence, SGLTl is a mem- ber of a large farnil!, of memhrune transport proteins. The sequence of SGLTI in m:unmals is highl>~ consen~ecl in both the small intestine and kiclne!- (Table 1 ). The amino terminus does not contain ;I reztdily identitiahle signal sequence [ 3.1, suggesting that it resides on the c-j.- toplasmic face of the membrane. This is supported h!r the fact that the protein is glycos~lated ;it Asn?tH lying in the hydrophilic domain hemeen transmembrrune segments 5 and 6 [3-j, I~iochcmical experiments indicate that major post-translational processing produces N-linked gl!,c~s)~- lation of either the tri or tettxantennary comples gpe ;it Asn2+8, and this contributes approsimatel!. 15 000 to the mass of the mature ~iiemlxine protein 1-t 1. Perh:ips sur- prisingly, gl~cosylation is not required for transport acti\, in’ in either ooc~~s or brush horder ~wmlxmes. There is ;1 second canonical amino-linked glycos~lation site ;II k~11306. WI the putative intracellular hydrophilic loop between transmemt~nlne segments 6 and 7, but this is not used [ 3.1, There are se\~~l consc’nsus sites for l~hos~~ho~lntion of SGLTl [i] 1 hut there is no c\%lence yet th:lt the!. pIa!. ;I functic~nal role. The) include sites on putative c-ytoplas- mic hydrophilic domains ( Ser303 :ind Ser-r 18) that ma!’ be recognized by CAMP-, cGMl’- and C- protein kineses. There are :llso two potenti: sites on external hydrophilic domains ( ThrSt and Seri66). hut these n~~ld he onI\. used if the seconda? structure moclel is incorrect or ;f there arc esternul kinascs. SGLTl has been functionall!~ expressed in .YvIIo~x~.< ooc~tes [ 1,~8,9**.10**], COS-7 cells [ 1 I 1, HeIa cells [ 111 and insect Sf9 cells [ 131. The substrate and ion specilicig’ and kinetics app’crlr to he quite independent of the expression system. This ~~)uld rule out impor tant effects of l”)st-t~lnslational moditicXions rind plasm:1 membrane composition on the t[‘:iiisporter function. The most comprehensive kinetic :lnalysis of SGLTl ws c;u- ried out hy Parent et trl. [9**.10**] using electrophysio- logical techniques. A six-stnte ordered kinetic model with mirror synimct~’ can cjuslitati\~el!. and clu:intitati\~el~ 3c‘- count for Na+ glucose cotl’;insport: this has heen hased on measurements of prestead!. st:itc currents. sugar~spe- cilic current-voltage (I V) cunw the voltage and Na+ dependence of sugar kinetics, the voltage ;ind sugar de- pendence of sodium kinetics. ;~nd the Na + leak currents Abbreviations GABA-y-amino butyrlc acid. @ Current Biology Ltd ISSN 0955-0674