5'-HETEROGENEITY IN HUMAN PROGESTERONE-RECEPTOR TRANSCRIPTS PREDICTS A NEW AMINO-TERMINAL TRUNCATED C-RECEPTOR AND UNIQUE A-RECEPTOR MESSAGES

Human progesterone receptors (PR) are thought to comprise two naturally occurring hormone-binding proteins: 94-kDa A-receptors and 120-kDa B-receptors. In this paper we present evidence for a third human PR, an N-terminally truncated, 45- to 50-kDa species, termed the C-receptor. To determine the translational origin of B- and A-receptors we mapped the multiple messages that code for human PR by Northern blot analyses, using a series of oligonucleotides and cDNA fragment probes corresponding to different regions of the PR message. In addition to the six transcripts of 2.5, 3.2, 4.5, 5.2, 6.1, and 11.4 kilobases (kb) originally described, we found that the 11.4-kb species is a complex of four bands that we have termed I-IV. Analysis of poly(A)+ RNA derived from T47D(V) human brest cancer cells using a variety of 5'-specific probes has identified three separate structural classes of human PR transcripts, indicating extensive 5'-termini heterogeneity. Class A messages, the 2.5- and 5.2-kb species, lack the sequences surrounding AUG(B) (codon 1), which is the translation initiation site for B-receptors, but contain AUG(A) (condon 165), the initiation site for A-receptors, and therefore, potentially encode only the latter. Class B messages, consisting of the 3.2-, 4.5, and 6.1-kb species as well as bands I and II of the 11.4-kb complex contain both AUG(B) and AUG(A) and could encode both receptor forms. Class C transcripts, bands III and IV in the 11.4-kb complex, lack AUG(B) and AUG(A) and, therefore, encode neither A- nor B-receptors, but contain down-stream sequences that hybridize to probes complementary to the DNA- and hormone-binding domains of PR. We propose that by utilization of an initiator methionine at condon 595 in exon 2, these messages direct the synthesis of a 45- to 50-kDa protein that lacks the N-terminus and first DNA-binding finger of PR, but contains the second DNA-binding finger, the hinge region, and the hormone-binding domain. This new human receptor, the C-receptor, appears to be abundantly expressed in progesterone target cells. (Molecular Endocrinology 4: 1833-1840, 1990)