Picturing Proteins

Reputations change in science. The nuclear matrix, the tangled web of proteins that gives the cell nucleus its bean-bag shape, was once considered by biochemists as a subcellular nuisance. Its insoluble mass made sorting out the real treasure in the nucleus -- the cell's DNA -- difficult.

But recently, thanks to technology such as the confocal microscope, the role of nuclear matrix has been re-evaluated, and is now seen as an integral part of the functioning of cellular genetic information. Scientists think the matrix provides architectural cues that guide where and when information is transmitted from genes in the nucleus' chromosomes.

Sharon Krauss of LBL's Life Sciences Division is making sense of nuclear matrix structure by investigating a cell component known as Protein 4.1. This protein, studied by Mohan Narla, Joel Chasis and John Conboy of LBL's Hematopoesis Group, is best understood as a structural protein of human red blood cells crucial for their characteristic "concave disk" shape and mechanical strength. Since red blood cells lack nuclei, most scientists previously had not considered that Protein 4.1 might play an important nuclear role. Krauss was the first to find Protein 4.1 in the nuclear matrix of human fibroblast cells. She believes it may play a structural role in the nucleus, perhaps at nuclear sites that are factories for DNA synthesis and RNA editing.

Imaging Protein 4.1 with the Confocal Microscope

Krauss visualized Protein 4.1 in nuclei using LBL's new confocal microscope with collaborator Carolyn Larabell. Invented three decades ago to probe connections between brain cells, the confocal microscope became widely used by scientists in the late 1980s. Today it is indispensable for investigating all aspects of cell biology from the makeup of cell organelles to the intricacies of protein scaffolding.

To create a cross-sectional picture of a cell, the confocal microscope pieces together thousands of tiny pinpoints taken one at a time with a laser. Since it collects light information one speck at time, the confocal microscope avoids much blurring seen in standard light microscopes.

Krauss, along with microscope experts Larabell and Steve Lockett, located Protein 4.1 in nuclear matrix by comparing its distribution to that of other known nuclear proteins. Proteins were highlighted with fluorescent antibodies of different colors for comparison. Starting from the rim of the nucleus, Krauss found Protein 4.1 inside rings of both nuclear membrane and its lamin protein lining. Protein 4.1 is in the central nuclear region also inhabited by another well known nuclear matrix protein.

Krauss is now using the confocal microscope to see if Protein 4.1 has some relationship to two types of factories residing in the nucleus--replisomes and spliceosomes. Replisomes are the protein machinery that copy the DNA in the nucleus, while spliceosomes edit RNA messages transcribed from DNA. These studies may reveal what role Protein 4.1 might have in the architecture and functioning of the cell nucleus.

-- Mike Wooldridge