The coat protein complex II (COPII) generates transport carriers that deliver newly synthesized proteins from the endoplasmic reticulum (ER) to the Golgi apparatus. The small GTPase Sar1 is a well-known regulator of the assembly of the COPII coat. In the present study, we demonstrate that, besides its well-established role in ER-to-Golgi trafficking, the nuclear localization of Sar1 is essential for the viability of Saccharomyces cerevisiae. Inhibition of either the nuclear entry or retention of Sar1 leads to a severe growth defect. Additionally, in vivo deletion of Sar1, by using conditional genetic depletion, further demonstrates that the loss of nuclear localization of Sar1 results in an abnormal nuclear envelope shape. Our findings highlighted a possible novel role of Sar1 within the nucleus, which may relate to the proper formation of the nuclear envelope.
Keywords: Sar1, COPII, small GTPase, nuclear envelope, membrane traffic

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In research on cell biology, organelles have been a major unit of such analyses. Researchers have assumed that the inside of an organelle is almost uniform in regards to its function, even though each organelle has multiple functions. However, we are now facing conundrums that cannot be resolved so long as we regard organelles as functionally uniform units. For instance, how can cells control the diverse patterns of glycosylation of various secretory proteins in the endoplasmic reticulum and Golgi in an orderly manner with high accuracy? Here, we introduce the novel concept of organelle zones as a solution; each organelle has functionally distinct zones, and zones in different organelles closely interact each other in order to perform complex cellular functions. This Copernican Revolution from organelle biology to organelle zone biology will drastically change and advance our thoughts about cells.

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Two decades have passed since the development of the first calcium indicator based on the green fluorescent protein (GFP) and the principle of Förster resonance energy transfer (FRET). During this period, researchers have advanced many novel ideas for the improvement of such genetically encoded FRET biosensors, which have allowed them to expand their targets from small molecules to signaling proteins and physicochemical properties. Although the merits of “genetically encoded” FRET biosensors became clear once various cell lines were established and several transgenic organisms were generated, the road to these developments was not necessarily a smooth one. Moreover, even today the development of new FRET biosensors remains a very labor-intensive, trial-and-error process. Therefore, at this junction, it may be worthwhile to summarize the progress of the FRET biosensor and discuss the future direction of its development and application.
Key words: FRET, biosensor, fluorescent protein

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