Genes encoding novel GTP‐binding proteins in Dictyostelium

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Abstract
We have identified a two‐member gene family in the Dictyostelium genome and have isolated corresponding cDNA or genomic DNA recombinant clones. Analyses of these DNA sequences predicted encoded proteins of ∼︁200 amino acids with ∼︁90% sequence identity to each other. These Dictyostelium proteins also share amino acids identity within the GTP‐binding domains in the family of G‐regulatory proteins involved in cellular regulation and transmembrane signalling. Additional structural similarities are seen with members of the ras supergene family, such as ras, ral, and rho. They are similar in size (usually ∼︁200 amino acids), possess four conserved domains involved in GTP interaction and are believed to be anchored in the membrane by fatty acid modification of a cysteine residue near the carboxy terminus. More extensive identity is observed with YPT1 and SEC4, two other members of this family of genes that are essential in yeast. The amino‐terminal half of both Dictyostelium proteins is 70% identical in amino acid sequence to the YPT1 and SEC4 yeast proteins with less identity continuing through the remainder of the proteins. In addition these proteins terminate in two cysteine residues that are thought to be required for membrane anchorage. The two genes within this Dictyostelium family are organized differently in the genome and are differentially regulated during development. One gene is colinear in sequence with its mRNA in the protein coding region, whereas the other gene encodes a spliced mRNA. The intron‐containing gene is associated with a developmentally regulated (AAC)‐repeat sequence. Finally, we have shown that the expression of one of the genes is induced during development with kinetics similar to that of other (AAC)n‐associated genes; conversely, the expression of the second gene is repressed at a similar developmental stage.