The Phenotypic Analysis of Lactobacillus plantarum shsp Mutants Reveals a Potential Role for hsp1 in Cryotolerance

Abstract

Small heat shock proteins (sHSPs) are ubiquitous, low molecular weight (MW) proteins that share a conserved alpha-crystallin domain. sHSPs oligomers exhibit chaperon-like activities by interacting with unfolded substrates, thereby preventing their aggregation and precipitation. Unlike most lactobacilli, which have single <i>shsp</i> genes, three different sHSP-encoding genes, i.e., <i>hsp1</i>, <i>hsp2</i>, and <i>hsp3</i>, were previously identified in the probiotic <i>Lactobacillus plantarum</i> WCFS1. Early studies, including the characterization of the knock out (KO) mutant for <i>hsp2</i>, indicated a different organization and transcriptional regulation of these genes and suggested that the three <i>L. plantarum</i> sHSPs might accomplish different tasks in stress response. To unravel the role of sHSPs, KO mutants of <i>hsp1</i> and <i>hsp3</i> were generated using a Cre-<i>lox</i> based system. Mutation of either genes resulted in impaired growth capacity under normal conditions, heat-stress and stresses typically found during host interactions and food technological process. However, survival to heat shock and the level of thermal stabilization of cytoplasmic proteins were similar between mutants and parental strain. Transcriptional analysis revealed that in the mutant genetic backgrounds there is an upregulated basal expression of the un-mutated mate <i>hsps</i> and other stress-related genes, which may compensate for the loss of HSP function, hence possibly accounting for the lack of a remarkable susceptibility to heat challenge. HSP3 seemed relevant for the induction of thermotolerance, while HSP1 was required for improved cryotolerance. Cell surface properties and plasma membrane fluidity were investigated to ascertain the possible membrane association of sHSP. Intriguingly, the loss of <i>hsp1</i> was associated to a lower level of maximal membrane fluidity upon heat stress. A role for HSP1 in controlling and improving membrane fluidity is suggested which may pertains its cryoprotective function.