Thus, we suggest that 4.1R is a member of the suite of direct cdc2 substrates that are required for the establishment of a bipolar spindle. INTRODUCTION Erythrocyte protein 4.1 (4.1R) is an 80-kDa cytoskeletal phosphoprotein critical to the dynamic organization of the spectrin/actin cytoskeleton and to the attachment of the cytoskeleton to the cell membrane (reviewed by Benz, 1994 ). direct cdc2 substrates that are required for the establishment of a bipolar spindle. INTRODUCTION Erythrocyte protein 4.1 (4.1R) is an 80-kDa cytoskeletal phosphoprotein critical to the dynamic organization of the spectrin/actin cytoskeleton and to the attachment of the cytoskeleton to the cell membrane (reviewed by Benz, 1994 ). The 80-kDa 4.1R of mature red blood cells is only one member of a large KU14R 4.1R family that arises from a single gene through the utilization of alternative promoters (Parra 2003 ), alternative pre-mRNA splicing (Conboy 1988 ; Tang 1990 ), and posttranslational modifications (Subrahmanyam 1991 ). Three 4.1R homologues, named 4.1G, 4.1N, and 4.1B, have been identified (Parra 1998 KU14R ; Walensky 1999 ; Yamakawa 1999 ; Ye 1999 ; Parra 2000 ). They share strong sequence homology with the 30-kDa membrane-binding domain (MBD), the 10CkDa spectrin/actin-binding (SAB) domain, and the 22C24-kDa C-terminal domain (CTD) of 4.1R. The NH2 terminus (head-piece or HP) and intervening sequences appear to be highly specific for each individual 4.1 gene. Several research studies are investigating the functional specificity and redundancy of these proteins. Isoforms of 4.1R are widely expressed in many tissues and are phylogenetically conserved (Cohen 1982 ; Granger and Lazarides, 1984 ), which imply their importance in cell function. Studies of 4.1R have revealed unexpected new roles for 4.1R proteins in various supramolecular structures in nucleated cells. In contrast to 4.1R’s strict peripheral localization in mature red blood cells, 4.1R isoforms have been shown to associate with interphase microtubules in human T-cells (Perez-Ferreiro 2001 ), contractile apparatus in skeletal muscles (Kontrogianni-Konstantopouloa 2000), and tight junctions in epithelial cells (Mattagajasingh 2000 ). Isoforms of 4.1R in the nucleus have also been shown to contribute to nuclear architecture (DeCarcer 1995 ; Krauss 1997a , 2002 , 2003 ; Mattagajasingh 1999 ) and splicing processes (Lallena and Correas, 1997 ). A unique feature of 4.1R is that it exhibits dynamic reorganization during the cell cycle (Mattagajasingh 1999 ; Krauss 1997a ). 4.1R localizes in the centrioles, nucleus, and cytoplasm of interphase cells, and in the spindle and spindle poles of mitotic cells. Previously, we have shown that a 135-kDa 4.1R isoform associates in a complex containing NuMA, cytoplasmic dynein, and dynactin (Mattagajasingh 1999 ), which is involved in the tethering of microtubules at the spindle poles and is essential for mitotic spindle-pole stabilization (Merdes 1996 ). 4.1R has also been shown to be crucial for in vitro mitotic aster assemblies in both egg extracts (Krauss 2004 ) and mammalian mitotic extracts (Huang 2004 ). The cell cycle-dependent localization of 4.1R and its association with microtubule-associated proteins strongly imply a critical role for 4.1R in the organization of the mitotic apparatus during cell division. However, little is known about the expression of 4.1R KU14R isoforms and the mechanisms by which it reorganizes during the cell cycle. The relocalization pattern of 4.1R during the cell cycle is similar to that of several KU14R proteins essential for mitotic spindle assembly, including NuMA (Price and Pettijohn, 1986 ), dynein (Steuer 1990 ), HSET (Kuriyama 1995 ), and Eg5 (Houliston 1994 ). Many of the cellular reorganizations Rabbit polyclonal to ADCYAP1R1 occurring at the onset of mitosis are.