For a long time I have been fascinated by how the compartments inside cells take on their specific shape and structure. The highly compartmentalized plant cell is a marvel of evolution. Each compartment inside the cell plays a specific role in the life of the cell, and helps to determine the structure and function of the cell as a whole. Studies of the proteins in each compartment tell us a great deal about what that compartment's function is. Understanding how specific proteins get to selected compartments inside of the cell was the focus of my postoctoral studies. My recent focus has been on how the ion calcium moves inside of the plant cell.

Recently my students and I have found a strong calcium pump located in the plant chloroplast thylakoid membrane. This pump functions in the light to concentrate calcium ions inside the thylakoid lumen. In this location calcium is available for assembly into the oxygen-evolving complex of photosystem II. The oxygen- evolving complex is responsible for photosynthetic oxygen production by plants and is the enzyme responsible for the 20% oxygen that is in our present atmosphere. The oxygen-evolving complex has a strict requirement for calcium and will not function unless calcium is available inside the thylakoid lumen. The calcium pump discovered in my lab at Gonzaga University provides a way for large amounts of calcium to cross the thylakoid membrane and accumulate in the thylakoid lumen.

In our studies we have also noted that large amounts of calcium appear to flow into the thylakoid lumen when light strikes a plant and that the calcium appears to be released by the thylakoid lumen when the lights are turned off. This is interesting because several key enzymes involved in photosynthetic carbon dioxide fixation are strongly inhibited by calcium. It seems likely that these enzymes are regulated in part by calcium flow across the thylakoid lumen. We are now preparing to measure the concentration of free calcium inside the thylakoid lumen in the light and in the dark. Through these studies we hope to determine the magnitude of calcium flow across the thylakoid membrane during the light dark cycle. These measurements will allow us to see if there is a correlation between calcium flow across the thylakoid membrane and the regulation of carbon dioxide fixation by plants.