Abstract
The halophilic Archaeon Halorubrum lacusprofundi, isolated from the perennially cold and hypersaline Deep Lake in Antarctica, was recently sequenced and compared to 12 Haloarchaea from temperate climates by comparative genomics. Amino acid substitutions for 604 H. lacusprofundi proteins belonging to conserved haloarchaeal orthologous groups (cHOGs) were determined and found to occur at 7.85% of positions invariant in proteins from mesophilic Haloarchaea. The following substitutions were observed most frequently: (a) glutamic acid with aspartic acid or alanine; (b) small polar residues with other small polar or non-polar amino acids; (c) small non-polar residues with other small non-polar residues; (d) aromatic residues, especially tryptophan, with other aromatic residues; and (e) some larger polar residues with other similar residues. Amino acid substitutions for a cold-active H. lacusprofundi β-galactosidase were then examined in the context of a homology modeled structure at residues invariant in homologous enzymes from mesophilic Haloarchaea. Similar substitutions were observed as in the genome-wide approach, with the surface accessible regions of β-galactosidase displaying reduced acidity and increased hydrophobicity, and internal regions displaying mainly subtle changes among smaller non-polar and polar residues. These findings are consistent with H. lacusprofundi proteins displaying amino acid substitutions that increase structural flexibility and protein function at low temperature. We discuss the likely mechanisms of protein adaptation to a cold, hypersaline environment on Earth, with possible relevance to life elsewhere.
Highlights
The surface of Earth is mostly covered by salty oceans, 90% of which are at temperatures of 5uC or lower
Preliminary analysis of the 604 conserved haloarchaeal orthologous groups (cHOGs) confirmed that the proteins are very similar in molecular weight (MW) and isoelectric point (Table 1)
Profiles of the mesophilic proteins were constructed for each protein family and aligned with the orthologous H. lacusprofundi proteins, and the identity of residues varying in the cold-adapted species that were invariant in the mesophilic sequences (5,541 residues out of 70,589 total) were extracted
Summary
The surface of Earth is mostly covered by salty oceans, 90% of which are at temperatures of 5uC or lower. Halorubrum lacusprofundi, a member of the ancient class of microorganisms in the Domain Archaea, is one of the few cold and salt-adapted species available in pure culture [10]. This microbe was isolated from Deep Lake, Antarctica where temperatures remain below 0uC for 8 months of the year. Due to the extremely high salinity of Deep Lake, reported as , 3.5 M NaCl, the freezing point is greatly depressed and the lake remains liquid throughout the year, even when temperatures reach a minimum of 218uC during the winter. H. lacusprofundi is a psychrotolerant microorganism capable of growth at sub-zero temperatures, while its optimum growth temperature is 30uC, lower than most other related haloarchaeal microorganisms (Table 1) [11,12]
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