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Abstract
The lamin B receptor (LBR) is a multi-spanning membrane protein of the inner nuclear membrane that is often employed as a “reporter” of nuclear envelope dynamics. We show here that the diffusional mobility of full-length LBR exhibits significant regional variation along the nuclear envelope, consistent with the existence of discrete LBR microdomains and the occurrence of multiple, asymmetrically-spaced anastomoses along the nuclear envelope-endoplasmic reticulum interface. Interestingly, a commonly used fusion protein that contains the amino-terminal region and the first transmembrane domain of LBR exhibits reduced mobility at the nuclear envelope, but behaves similarly to full-length LBR in the endoplasmic reticulum. On the other hand, carboxy-terminally truncated mutants that retain the first four transmembrane domains and a part or the whole of the amino-terminal region of LBR are generally hyper-mobile. These results suggest that LBR dynamics is structure and compartment specific. They also indicate that native LBR is probably “configured” by long-range interactions that involve the loops between adjacent transmembrane domains and parts of the amino-terminal region.
Highlights
The lamin B receptor (LBR) is an integral protein of the inner nuclear membrane originally identified by in vitro binding to lamin B [1,2,3]
To analyze the dynamic properties of LBR, we engineered 16 eGFP-fusion proteins that correspond to full-length LBR (FL-LBR) or LBR mutants (S1A Fig)
FL-LBR co-localized to a large extent with nuclear lamin B, but its distribution pattern of was distinct from that of BiP and Nup84/107 (Fig 1B)
Summary
The lamin B receptor (LBR) is an integral protein of the inner nuclear membrane originally identified by in vitro binding to lamin B [1,2,3]. The LBR molecule contains eight (predicted) transmembrane domains. These domains exhibit high sequence homology to fungal and animal sterol reductases [4] and are flanked by two hydrophilic end-pieces at the amino- and carboxy-terminal ends [5,6,7]. The protein diffuses laterally across the ER, reaches the outer nuclear mebrane, passes through the nuclear pore complexes (NPCs) and arrives at the inner nuclear membrane, where is “trapped” by binding to nuclear lamina and peripheral heterochromatin
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