Isolation and Characterization of Four Coccidioides immitis Septin (SEP) Genes

Alejandro Nila, Garry Cole


Septins are a family of conserved proteins that have been implicated in a variety of cellular functions. These include such biological processes as cell polarity and septation. Members of the septin gene family have been found in a range of eukaryotic organisms except for members of the plant kingdom. Four septin gene homologs designated SEP1, SEP2, SEP3 and SEP4 have been cloned from the human pathogenic fungus, Coccidioides immitis. Southern blot hybridization has shown that these are single copy genes. Based on cDNA and genomic sequence analyses, the predicted CiSep1 protein has a molecular weight of 43.8 kDa, shares three conserved motifs (G1, G2 and G3) which are structurally similar to those found in GTPases (G1 [GXXXXGKS/T], G3 [DXXG] and G4 [XKXD]), and contains a coiled-coil domain at its carboxyl terminus as predicted by computational analysis. CiSep2 has a deduced molecular weight of 42.9 kDa, shares the G1, G3 and G4 motifs of GTPases, and also contains the coiled-coil domain at its C-terminus. CiSep3 and CiSep4 have a predicted MW of 39.2 kDa and 45.4 kDa, respectively, share the GTPase motifs, but lack the coiled-coil domain. The deduced CiSep1 amino acid sequence shows 89.0% similarity to the AspA septin of Aspergillus nidulans. The protein sequence of CiSep2 reveals 92.2% similarity to AspB, CiSep3 shows 86.6% similarity to AspD, and CiSep4 demonstrates 59.0% similarity to AspE. Based on results of construction of a phylogenetic tree, the CiSep1, CiSep2, CiSep3 and CiSep4 septins are accommodated in the Cdc11, Cdc3, Cdc10 and Cdc11 classes of yeast cell division cycle proteins, respectively. Reverse transcription/real-time polymerase chain reaction (PCR) assays showed that the four C. immitis septins are differentially expressed during maturation of the saprobic phase of the pathogen. The septins of C. immitis consist of a family of at least four genes which belong to three orthologous classes and show structural homology to septins of other ascomycetous fungi. In spite of the high homology demonstrated by these genes, their functions in filamentous fungi are still unknown.

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American Society for Microbiology General Meeting
    • ASM 102nd (2002)