The putative type I secretion sequence of a 1.5-MDa icebinding adhesin folds as a Ca2+-dependent β-rich structure

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Tony He


Marinomonas primoryensis is a Gram-negative and strictly aerobic bacterium, an isolate of which was found in a brackish (half-strength seawater) lake in Antarctica. M. primoryensis produces a Ca2+-dependent 1.5-MDa ice-binding adhesin (MpAFP) that likely binds the bacterium to the underside of surface ice, where it may have better access to oxygen and other nutrients. The MpAFP is divided into five distinct regions that include the highly repetitive Region II (RII) and the moderately repetitive ice-binding Region IV (RIV). Region V (RV) is the C-terminal domain of MpAFP, and may serve as its non-cleavable signal sequence for the Type I Secretion System (TISS). The Protein Homology/analogY Recognition Engine (Phyre2) Server modeled RV as a Ca2+-bound β-rich structure. A previous study used a RV construct that started directly following the C-terminus of RIV, but the expressed protein aggregated and was unsuitable for further characterization. Here, we report the expression and purification of a fusion construct spanning RIV and RV. We defined the start of the RV domain by digesting the purified RIV-V fusion protein with trypsin in the presence of Ca2+. A soluble RV was then purified from the digestion mixture using anion-exchange chromatography, and its N-terminal residues were determined using Edman degradation sequencing. The folding of the newly designed RV construct was assessed by size-exclusion chromatography and circular dichroism (CD) spectroscopy. This work will provide insight into the structural relationship between RIV and RV, as well as how extremely large proteins are secreted via the TISS in Gram-negative bacteria.

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Author Biography

Tony He, Queen's University, Kingston, Ontario, Canada.

Supervised by: Peter Davies, Queen’s University