LIFETIME ANALYSIS OF FLUORESCENT PROTEIN VARIANTS

该子项目是利用NIH/NCRR资助的中心赠款提供的资源的许多研究子项目之一。子项目和研究者（PI）可能从另一个NIH来源获得了主要资金，因此可以在其他CRISP条目中表示。所列机构为中心，不一定是研究者所在机构。我的实验室已经开发了天然四聚体红色荧光珊瑚蛋白的单体形式（坎贝尔等人（2002）PNAS 99：7877）。这种“mRFP 1”非常受欢迎，因为它可以与各种其他蛋白质进行基因融合，将它们标记为红色，而不会导致它们聚集，沉淀或其他错误的交通。然而，mRFP 1确实有一个显著的缺点：它的荧光量子产率仅为0.25，而天然四聚体高出约3倍。我们试图通过各种诱变和定向进化方法来增加量子产率，但由于缺乏高通量（优选基于FACS的）量子产率测定而受挫。测量整体亮度或强度非常容易（我们有B-D FACS DiVa），但这反映了功能蛋白质拷贝数X消光系数X量子产率的乘积。在基于mRFP 1的随机文库中对最亮的细胞进行分选通常会发现高表达的蛋白质，偶尔会发现消光系数增加的突变体，并且迄今为止从未产生量子产率的增加。但是我们已经注意到量子产率和激发态寿命之间的粗略相关性，尽管我们没有太多的数据点。mRFP 1的QY为0.25与tau约1.8 ns相关，而其二聚体和四聚体前体的QY分别为0.68和0.79，tau分别为3.8和4.0 ns。当然，教科书预测，对于给定的自然辐射寿命固定但淬灭可变的发色团，量子产率和实际寿命应该彼此成正比。以上三个例子离理论上的比例并不太远。如果我们可以对激发态寿命值为1.8 ns的单个细胞进行分类，也许我们会对量子产率进行分类-这至少应该与蛋白质表达水平和消光系数无关。本项目的目标是评估使用相敏流式细胞术测量荧光寿命来识别荧光蛋白的明亮变体的可行性。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. My lab has developed a monomeric version of a naturally tetrameric red fluorescent coral protein (Campbell et al (2002) PNAS 99: 7877). This "mRFP1" has been very popular because it can be genetically fused to a wide variety of other proteins to label them red without causing them to aggregate, precipitate, or otherwise mis-traffic. However, mRFP1 does have a significant drawback: its fluorescence quantum yield is only 0.25 whereas the natural tetramer is about 3 fold higher. Our attempts to increase the quantum yield by various mutagenic and directed evolution approaches have been frustrated by the lack of a high-throughput (preferably FACS-based) assay for quantum yield. It is very easy to measure overall brightness or intensity (we have a B-D FACS DiVa), but this reflects the product of number of functional protein copies X extinction coefficient X quantum yield. Sorting for the brightest cells in a randomized library based on mRFP1 usually finds highly expressing proteins, occasionally finds mutants with increased extinction coefficients, and has so far never produced an increase in quantum yield. But we have noticed a rough correlation between quantum yield and excited state lifetime, though we don't have many data points. mRFP1's QY of 0.25 correlates with a tau of about 1.8 ns, whereas its dimeric and tetrameric predecessors have QYs of 0.68 and 0.79 and tau's of 3.8 and 4.0 ns respectively. Of course, the textbooks predict that quantum yield and actual lifetime should be directly proportional to each other for a given chromophore of fixed natural radiative lifetime but variable quenching. The above three examples are not too far from the theoretical proportionality. If we could sort single cells for excited state lifetime for values 1.8 ns, perhaps we would be sorting for quantum yield -- this should at least be independent of protein expression level and extinction coefficient. The goal of this project is to evaluate the feasibility of identifying bright variants of fluorescent proteins using phase sensitive flow cytometry measurements of fluorescence lifetime.