In 1972, Wu & Stryer estimated the size and shape of bovine rhodopsin, a light-s
ID: 224129 • Letter: I
Question
In 1972, Wu & Stryer estimated the size and shape of bovine rhodopsin, a light-sensing protein found in the rod cells of the eye, using fluorescence resonance energy transfer (FRET). A major question at the time was whether such a small protein (29 kDa) could traverse the 75 A disc membrane. Three sites (A-C) were labeled with fluorophores, and transfer efficiency (E) was measured between labels and between a label and the bound retinal cofactor, resulting in the following data: Calculate distances between all six pairs. A spherical protein with the same molecular weight of rhodopsin would have a diameter of 45 A. What can you say about the shape of rhodopsin? Is it possible for rhodopsin to traverse the disc membrane of a rod cell?Explanation / Answer
a. Calculations of the distances between 6 pairs are –
1. In case of A distance is –
E = [R06 / (R06 + R 6)] , R6 = 1.785 X 1011
So, R = (1.785 x 1011)1/6 = 75
2. In case of B, distance is – R = 57
3. In case of C, the distance is –R = 46
4. In case of A, the distance is – R = 35
5. In case of A, the distance is – R = 32
6. In case of B, the distance is – R = 31
b. If the diameter of rhodopsin is 45Å, it suggests that the shape of rhodopsin would be spherical.
c. It is possible for the spherical rhodopsin to traverse the disc membrane of rod cell.