Functional characterization of the CATS gene with respect to its role in normal hematopoiesis and in leukemia

The Clathrin Assembly Lymphoid Myeloid leukemia gene (CALM) was
first identified as the fusion partner of AF10 in the
t(10;11)(p13;q14) translocation. The CALM/AF10 fusion protein plays
a crucial role in t(10;11)(p13;q14) associated leukemogenesis.
Using the N-terminal half of CALM as a bait in a yeast two-hybrid
screen a novel protein named CATS (CALM interacting protein
expressed in thymus and spleen) was identified as CALM interacting
partner. Multiple tissue Northern blot analysis showed predominant
expression of CATS in lymphoid tissues. CATS codes for two protein
isoforms of 238 and 248 amino acids. The interaction between CALM
and CATS was confirmed by co-immunoprecipitation and colocalization
experiments. The CATS interaction domain of CALM was mapped to
amino acids 221 to 294 of CALM. This domain is contained in the
CALM/AF10 fusion protein. CATS localizes to the nucleus and shows a
preference for nucleoli. Expression of CATS was able to markedly
increase the nuclear localization of CALM and of the leukemogenic
fusion protein CALM/AF10. This effect of CATS seems to be stronger
on CALM/AF10 than on CALM. Several monoclonal antibodies against
the C-terminus of human CATS were generated. These antibodies
recognize both the human and the murine CATS protein. Western blot
analyses showed that CATS is strongly expressed in different human
leukemia, lymphoma and tumor cell lines but not in resting T-cells.
High CATS expression in proliferating cells as well as its
nucleolar localization suggest a role of CATS in the control of
cell proliferation. In order to gain further insight into CATS
function we used CATS as a bait in a yeast two-hybrid screen.
Several CATS interacting proteins with apparently unrelated
function were identified. Interestingly, on closer scrutiny these
proteins could be associated with three key regulatory pathways:
signaling, apoptosis and cell cycle control. We discuss in detail
the biological relevance of the CATS interaction with the two
apoptosis-associated proteins HAX1 and SIVA, the cell cycle
regulator KIS and the CALM interacting ribonucleoprotein PCBP1. Our
results indicate that the subcellular localization of CALM and
CALM/AF10 could depend in part on the presence of CATS with a
greater fraction of CALM or CALM/AF10 being present in the nucleus
of cells with high CATS expression (e.g. lymphoid cells have high
CATS expression). Moreover we provide evidences that CATS function
might be tightly linked to cancer initiation and/or progression.
The CALM-CATS interaction might thus play an important role in
CALM/AF10 mediated leukemogenesis.