Candida albicans SUR7 contributes to secretion, biofilm formation, and macrophage killing
BMC Microbiology
RCeaseanrcdhairdticale albicans SUR7 contributes to secretion, biofilm formation, and macrophage killing
Stella M Bernardo 0 1
Samuel A Lee 0 1
0 Section of Infectious Diseases, New Mexico Veterans Healthcare System , Albuquerque, NM , USA
1 Division of Infectious Diseases, Department of Medicine, University of New Mexico Health Science Center , Albuquerque, NM , USA
Background: Candida albicans SUR7 has been shown to be required for plasma membrane organization and cell wall synthesis, but its role in virulence is not known. Using a bioinformatics strategy, we previously identified several novel putative secretion pathway proteins potentially involved in virulence, including the C. albicans homolog of the Saccharomyces cerevisiae endocytosis-related protein Sur7p. We therefore generated a C. albicans sur7 null mutant and examined its contribution to key virulence attributes. Results: Structurally, the C. albicans sur7 mutant was impaired in response to filamentation-inducing conditions, and formed aberrant hyphae with extensive accumulation of plasma membrane-derived structures within the cell. Absence of SUR7 resulted in a temperature-sensitive growth defect at high temperatures (42C), which was partially rescued by addition of NaCl. We next examined the role of the SUR7 paralog C. albicans FMP45 in this temperaturesensitive phenotype. Analysis of C. albicans Fmp45p-GFP demonstrated co-localization of Fmp45p with Sur7p and increased fluorescence in the plasma membrane in the presence of high salt. We next focused on key virulence-related phenotypes. The C. albicans sur7 null mutant exhibited secretory defects: reduced lipase secretion, and increased levels of secreted Sap2p. The null mutant was hyper-susceptible to sub-inhibitory concentrations of caspofungin, but not amphotericin B and 5-fluorocytosine. Functionally, the sur7 mutant demonstrated increased adhesion to polystyrene and of note, was markedly defective in biofilm formation. In an in vitro macrophage model of virulence, the sur7 mutant was impaired in macrophage killing. Conclusions: Plasma membrane and cell wall organization are important for cell morphology, and alterations of these structures contributed to impairment of several key virulence-associated phenotypes in the C. albicans sur7 mutant.
-
Background
C. albicans SUR7 shares 44% identity and 65% similarity
with S. cerevisiae SUR7. S. cerevisiae SUR7 encodes a
predicted integral membrane protein with an N-terminal
signal sequence and four transmembrane domains, and is
a member of a family of proteins that also includes
Yn1194p, Ydl222p, and Ylr414cp [1,2]. Sur7p localizes to
large, immobile, stable cortical patches on the plasma
membrane, termed "eisosomes" which mark sites of
endocytosis [3,4]. Deletion of S. cerevisiae SUR7 resulted
in a strain with a defect in sporulation and altered plasma
membrane sphingolipid content [4].
Alvarez and Konopka [5] identified C. albicans Sur7p in
a detergent-resistant fraction of the plasma membrane in
a proteomics study on N-acetylglucosamine-induced
proteins. Recently, they generated a C. albicans sur7
knockout mutant which is characterized by aberrant cell wall
organization [2]. Specifically, lack of SUR7 in C. albicans
results in mislocalization of actin and septin, and
abnormal cell wall material protruding into and forming
structures within the cytoplasm. However, from a phenotypic
standpoint, little is known regarding the role of C.
albicans SUR7 in pathogenesis.
A number of C. albicans virulence-related secreted
proteins that remain associated with the plasma
membrane or cell wall have been identified, including the
outer mannoprotein Hwp1p [6], adhesins encoded by the
ALS family of genes [7], and membrane proteins encoded
by the pH-responsive genes PHR1 and PHR2 [8-11].
However, a genome-wide understanding of Candida
secretory pathway proteins and virulence is still limited.
Previously, we took advantage of SignalP v2.0 [12,13] and
a series of additional validated predictive algorithms to
define a computational secretome of C. albicans from its
entire genome [14]. In addition to identifying putative
soluble secretory proteins, we also identified a number of
putative and known membrane and cell-wall associated
proteins [14]. We next compared these databases with
published genome-wide expression profiling data to
identify candidate virulence-related genes. Fradin et al. [15]
performed genomic expression profiling in C. albicans
exposed in vitro to blood and in vivo during infection in a
standard mouse model of disseminated candidiasis and
identified groups of genes highly expressed under these
conditions. When compared with the dataset of
predicted secretion pathway ORFs, a number of
virulencerelated genes were concordant, including Hwp1p and the
Als family of adhesins [6,7], Phr1p [8], Sap9p [16], Sod5p
[17,18], and Sun41p [19-21]. Thus, we identified known
soluble secreted and membrane-associated secretion
pathway (...truncated)