8.A.109. The Endoplasmic Reticulum Junction-forming protein (Lunapark) Family
Lnp1p, a member of the conserved Lunapark family, plays a role in ER network formation in yeast. Lnp1p binds to the reticulons and Yop1p and resides at ER tubule junctions in both yeast and mammalian cells. In the yeast Saccharomyces cerevisiae, the interaction of Lnp1p with the reticulon protein, Rtn1p, and the localization of Lnp1p to ER junctions are regulated by Sey1p, the yeast orthologue of atlastin. Chen et al. 2012 proposed that Lnp1p and Sey1p act antagonistically to balance polygonal network formation. In support of this proposal, they showed that the collapsed, densely reticulated ER network in lnp1 Δ cells is partially restored when the GTPase activity of Sey1p is abrogated.
Mammalian Lnp1 (mLnp1) affects ER junction mobility and hence network dynamics. Three-way junctions with mLnp1 are less mobile than junctions without mLnp1. Newly formed junctions that acquire mLnp1 remain stable within the ER network, whereas nascent junctions that fail to acquire mLnp1 undergo rapid ring closure. Thus, mLnp1 plays a key role in stabilizing nascent three-way ER junctions (Chen et al. 2015). Moreover, Lnp1 maintains nucleopore stability and plays a role in NPC integrity, separate from its functions in the ER. It may be linked to Ndc1 and Rtn1 interactions (Casey et al. 2015). In plants (A. thaliana), AtLNP1 and AtLNP2 are involved in determining the network morphology of the plant ER, possibly by regulating the formation of ER cisternae (Kriechbaumer et al. 2018).
The ER junction-forming protein, Lunapark, LNP1 of 278 aas and possibly 2 N-terminal TMSs or 4 TMSs in a 2 + 2 arrangement with two N-terminal and 2 more C-terminal (Chen et al. 2012). LNP1 aids in membrane fusion and maintenance of the endoplasmic reticulum (Casey et al. 2015). Works in conjunction with the ER shaping proteins (reticulons RTN1 and RTN2 (TC# 8.A.102), YOP1), and in antagonism to SEY1 to maintain the network in a dynamic equilibrium. May counterbalance SEY1-directed polygon formation by promoting polygon loss through ring closure (Chen et al. 2012).
LNP1 of Saccharomyces cerevisiae
Lunapark LNP1 of 408 aas and 2 N-terminal TMSs. Kriechbaumer et al. 2018 identified two Arabidopsis LNP homologues and investigated their subcellular localization via confocal microscopy and potential function in shaping the ER network using protein-protein interaction assays and mutant analysis. They showed that AtLNP1 overexpression in tobacco leaf epidermal cells mainly labels cisternae in the ER network, whereas AtLNP2 labels the whole ER. Overexpression of LNP proteins resulted in an increased abundance of ER cisternae. AtLNP1 and AtLNP2 appear to be involved in determining the network morphology of the plant ER, possibly by regulating the formation of ER cisternae (Kriechbaumer et al. 2018).
LNP1 of Arabidopsis thaliana
At LNP2 of 409 aas and 2 N-terminal TMSs (Kriechbaumer et al. 2018).