The subunit composition of an oligomeric protein can be determined by treating the protein with a cross-linking agent (a bifunctional molecule that reacts with and links groups in two different portions of the polypeptide), denaturing the protein, and analyzing the products by SDS-PAGE (Section 5-2D).
(a) An oligomer analyzed by SDS-PAGE shows a single 40 kD polypeptide. Brief treatment with a cross-linking agent yields the SDS-PAGE banding pattern shown below. What is the protein's probable subunit composition?

(b) Not shown in the electrophoretogram above are faint bands at 240 kD and 360 kD. What do these bands represent?
(c) Another protein analyzed by SDS-PAGE shows two polypeptides at 20 kD and 50 kD. Chemical cross-linking of the native protein yields the results shown below in SDSPAGE. Not shown is a very faint band at 540 kD. What is the subunit composition of this protein?

Transcribed Image Text:

120 kD -40 kD 270 kD 180 kD 50 kD - 20 kD