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bioRxiv · 10.1101/2021.02.16.431378

N-terminal extensions strengthen hydrophobic inter-subunit interactions between HU's C-terminal domains to frustrate heterodimer formation

Arora, K.·Thakur, B.·Mrigwani, A.·Guptasarma, P.
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Abstract

HU is a nucleoid-associated protein (NAP) that helps bacterial chromosomal DNA to remain compact. Escherichia coli contains two homologs of HU that are ~ 70 % identical: HU-A and HU-B. The early log phase, late log phase, and stationary phase of E. coli growth are reported to be dominated, respectively, by HU-AA homodimers, HU-BB homo-dimers, and HU-AB heterodimers. Here, we show that the formation of HU-AB heterodimers occurs to a much lower degree in HU chains that have a displaced N-terminus, whether through addition of an N-terminal affinity (polyhistidine) tag, or fusion of a fluorescent protein. A combination of mass spectrometry, spectroscopy, chromatography, and electrophoresis (exploring glutaraldehyde crosslinking of subunits) was used to study the mixing, co-expression, unfolding and refolding of HU-AA and HU-BB homodimers. The data suggests that, in HU polypeptides with N-terminal extension, whereas inter-subunit contacts between the alpha helical N-terminal domains (NTDs) undergo facile unfolding and dissociation, inter-subunit contacts between the beta sheet- and IDR-dominated C-terminal domains (CTDs) fail to do so, due to persistence of hydrophobic inter-subunit interactions between two beta sheets. This persistence causes HU to remain nominally dimeric even after substantive unfolding, and frustrates subunit exchange and heterodimer formation.

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Arora, K., Thakur, B., Mrigwani, A., Guptasarma, P.. 2021-02-17. N-terminal extensions strengthen hydrophobic inter-subunit interactions between HU's C-terminal domains to frustrate heterodimer formation. https://doi.org/10.1101/2021.02.16.431378

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