My question produced a stream of answers/suggestions. Many thanks to those who
answered. To briefly summarise:
Several people suggested that 1% Triton was extracting intrinsic membrane
proteins: this is not the case - although the membranes are broken up, they are
still in *suspension* rather than solution, and are pelleted by centrifugation.
The Triton does extract all the chlorophyll, though.
Other suggestions included: peroxidase, acid phosphatase or aldolase. However,
none of these would be present in the sort of quantities I am seeing.
The answer is, of course, blindingly obvious - "ACTIN !!!" (this was the full
text of the answer from prizewinner David Lonsdale (LONSDALE at AFRC.JII). No
other soluble protein could conceivably compete with RUBISCO is terms of
PS - the 50kDa band was one subunit of the chloroplast ATPase CFo, which the
Triton presumably *does* strip off the membrane.