RNA Detection

[8, 9]. Endosomal movement along microtubules is mediated by

the concerted action of plus-end directed kinesin-3 type motor

Kin3 and minus-end directed cytoplasmic dynein Dyn1/2 [10].

The associated transport machinery distributes mRNAs and ribo-

somes throughout the hyphae to avoid aberrant gradients of

mRNAs and translated proteins around the nucleus [11, 12].

Importantly, endosomal transport of septin mRNAs and encoded

proteins is needed for assembly of heteromeric septin complexes

and their delivery and incorporation into higher-order filaments at

the growth pole of hyphae (Fig.1a;[13, 14]).

Key players for mRNA transport are RNA-binding proteins

(RBPs) that recognize specific cis-acting elements within the

cargo mRNAs [15, 16]. The so-called RNA zip codes can by

recognized by their primary sequence or their more complex ter-

tiary structures. Recent evidence revealed that RBPs act in concert

to increase affinity and specificity for the interaction with distinct

RNA elements forming a higher-order structure called mRNP

(mRNA ribonucleoprotein complex; [6, 16]). The key RNA-

binding protein of septin mRNA transport inU. maydisis Rrm4,

which contains three N-terminal RNA recognition motifs for RNA-

binding and a C-terminal MademoiseLLE domain for interaction

with the adaptor-like protein Upa1 [8, 17–19]. The latter connects

Fig. 1Concept of RNA live imaging inU. maydis.(a) Schematic representation illustrating endosomal mRNA
transport including the RNA live imaging concept inU. maydis.cdc3mRNA (blue) is bound by the RNA binding
protein Rrm4. In the 3^0 UTR stem-loops are integrated that are bound by RNA-binding proteins tagged with
fluorescence proteins (green). The endosomal transport machinery is shown ingray(Rab5a, small G protein;
Yup1, SNARE; Hok1, motor adaptor; Kin3, plus-end directed kinesin; septins Cdc3, Cdc10, Cdc11, and Cdc12
are indicated as bean-like shapes). (b) Scheme of RNA live imaging depicting mRNA with stem-loops in 3^0
UTR. These stem-loops are recognized by an RNA-binding protein (RBP,red) coupled to a fluorescence protein
(FP,green). (c) Comparison of three different systems for the visualization of mRNA: in theλN system the RBP
(λN*;brown) binds as a monomer, in the PP7 system the RBP (PCP;pink) binds as a dimer and in the MS2
system the RBP (MCP;light blue) is already expressed as a synthetically fused tandem dimer

320 Sabrina Zander et al.